You should know that all acts of disobedience are poison to the heart and cause its sickness and ruin. They result in its will running off course, against that of Allah, and so its sickness festers and increases. Ibn al-Mubarak said:
I have seen wrong actions killing hearts,
And their degradation may lead to their becoming addicted to them.
Turning away from wrong actions gives life to the hearts,
And opposing your self is best for it.
Whoever is concerned with the health and life of his heart, must rid it of the effects of such poisons, and then protect it by avoiding new ones. If he takes any by mistake, then he should hasten to wipe out their effect by turning in repentance and seeking forgiveness from Allah, as well as by doing good deeds that will wipe out his wrong actions.
By the four poisons we mean unnecessary talking, unrestrained glances, too much food and keeping bad company. Of all the poisons, these are the most widespread and have the greatest effect on a heart's well-being.
It is reported in al-Musnad, on the authority of Anas, that the Prophet said: "The faith of a servant is not put right until his heart is put right, and his heart is not put right until his tongue is put right." This shows that the Prophet has made the purification of faith conditional on the purification of the heart, and the purification of the heart conditional on the purification of the tongue.?
At-Tirmidhi relates in a hadith on the authority of Ibn 'Umar:??? "Do not talk excessively without remembering Allah, because such excessive talk without the mention of Allah causes the heart to harden, and the person furthest from Allah is a person with a hard heart."? 
'Umar Ibn al-Khattab, may Allah be pleased with him, said:??? "A person who talks too much is a person who often makes mistakes, and someone who often makes mistakes, often has wrong actions. The Fire has a priority over such a frequent sinner."
In a hadith related on the authority of Mu'adh, the Prophet said, "Shall I not tell you how to control all that?" I said, "Yes do, O Messenger of Allah." So he held his tongue between his fingers, and then he said:??? "Restrain this." I said, "O Prophet of Allah, are we accountable for what we say?" He said,??? "May your mother be bereft by your loss! Is there anything more than the harvest of the tongues that throws people on their faces (or he said "on their noses") into the Fire?" 
What is meant here by "he harvest of the tongues'" is the punishment for saying forbidden things. A man, through his actions and words, sows the seeds of either good or evil. On the Day of Resurrection he harvests their fruits. Those who sow the seeds of good words and deeds harvest honour and blessings; those who sow the seeds of evil words and deeds reap only regret and remorse.?
A hadith related by Abu Huraira says, "What mostly causes people to be sent to the Fire are the two openings: the mouth and the private parts."?
Abu Huraira also related that the Messenger of Allah said, "The servant speaks words, the consequences of which he does not realise, and for which he is sent down into the depths of the Fire further than the distance between the east and the west."?
The same hadith was transmitted by at-Tirmidhi with slight variations:? "The servant says something that he thinks is harmless, and for which he will be plunged into the depths of the Fire as far as seventy autumns."??? ?
Uqba ibn Amir said: "I said: "O Messenger of Allah, what is our best way of surviving?" He, may Allah bless him and grant him peace, replied:??? "Guard your tongue, make your house suffice for sheltering your privacy, and weep for your wrong actions.""?
It has been related on the authority of Sahl ibn Sa'd that the Prophet said,? "Whoever can guarantee what is between his jaws and what is between his legs, I guarantee him the Garden."
It has also been related by Abu Huraira, may Allah be pleased with him, that the Prophet, may Allah bless him and grant him peace, said,??? "Let whoever believes in Allah and the Last Day either speak good or remain silent."
Thus talking can either be good, in which case it is commendable, or bad, in which case it is haram.
The Prophet said:??? "Everything the children of Adam say goes against them, except for their enjoining good and forbidding evil, and remembering Allah, Glorious and Might is He."? This was reported by at-Tirmidhi and Ibn Majah on the authority of Umm Habiba, may Allah be pleased with her.
Umar ibn al-Khattab visited Abu Bakr, may Allah be pleased with them, and found him pulling his tongue with his fingers. Umar said "Stop! may Allah forgive you!" Abu Bakr replied;??? "This tongue has brought me to dangerous places."
Abdullah ibn Masud said: "By Allah, besides Whom no god exists, nothing deserves a long prison sentence more than my tongue." He also used to say:??? "O tongue, say good and you will profit; desist from saying evil things and you will be safe; otherwise you will find only regret."
Abu Huraira reported that Ibn al-Abbas said: "A person will not feel greater fury or anger for any part of his body on the Day of Judgement more than what he will feel for his tongue, unless he only used it for saying or enjoining good."
Al-Hassan said: "Whoever does not hold his tongue cannot understand his deen."
The least harmful of a tongue's faults is talking about whatever does not concern it. The following hadith of the Prophet is enough to indicate the harm of this fault: "One of the merits of a person's Islam is his abandoning what does not concern him."
Abu Ubaida related that al-Hassan said: "One of the signs of Allah's abandoning a servant is His making him preoccupied with what does not concern him."
Sahl said, "Whoever talks about what does not concern him is deprived of truthfulness."
As we have already mentioned above, this is the least harmful of the tongue's faults. There are far worse things, like backbiting, gossiping, obscene and misleading talk, two-faced and hypocritical talk, showing off, quarrelling, bickering, singing, lying, mockery, derision and falsehood; and there are many more faults which can affect a servant's tongue, ruining his heart and causing him to lose both his happiness and pleasure in this life, and his success and profit in the next life. Allah is the One to Whom we turn for assistance.
The unrestrained glance results in the one who looks becoming attracted to what he sees, and in the imprinting of an image of what he sees in his heart. This can result in several kinds of corruption in the heart of the servant. The following are a number of them:
It has been related that the Prophet once said words to the effect:? "The glance is a poisoned arrow of shaytan. Whoever lowers his gaze for Allah, He will bestow upon him a refreshing sweetness which he will find in his heart??? on the day that he meets Him."
Shaytan enters with the glance, for he travels with it, faster than the wind blowing through an empty place. He makes what is seen appear more beautiful than it really is, and transforms it into an idol for the heart to worship. Then he promises it false rewards, lights the fire of desires within it, and fuels it with the wood of forbidden actions, which the servant would not have comitted had it not been for this distorted image.
This distracts the heart and makes it forget its more important concerns. It stands between it and them; and so the heart loses its straight path and falls into the pit of desire and ignorance. Allah, Mighty and Glorious is He, says:
'"And do not obey anyone whose heart We have made forgetful in remembering Us'- who follows his own desires, and whose affair has exceeded all bounds."??? (18:28)
The unrestrained gaze causes all three afflictions.
It has been said that between the eye and the heart is an immediate connection; if the eyes are corrupted, then the heart follows. It becomes like a rubbish heap where all the dirt and filth and rottenness collect, and so there is no room for love for Allah, relating all matters to Him, awareness of being in His presence, and feeling joy at His proximity - only the opposite of these things can inhabit such a heart. Staring and gazing without restraint is disobedience to Allah:
"Tell the believing men to lower their gaze and guard their modesty; that is more purifying for them.?
Surely Allah is aware of what they do." (24:30)
Only the one who obeys Allah's commands is content in this world, and only the servant who obeys Allah will survive in the next world.
Furthermore, letting the gaze roam free cloaks the heart with darkness, just as lowering the gaze for Allah clothes it in light. After the above ayah, Allah, the Glorious and Mighty, says in the same surah of the Qur'an:
"Allah is the light of the heavens and the earth: the likeness of His light is as if there were a niche, and in the niche is a lamp, and in the lamp is a glass, and the glass as it were a brilliant star, lit from a blessed tree, an olive, neither of the east nor of the west, whose oil is well nigh luminous, though fire scarce touched it. Light upon light. Allah guides whomever He wants to His Light. Allah strikes metaphors for man; and Allah knows all things."??? (24:35)
When the heart is a light, countless good comes to it from all directions. If it is dark, then clouds of evil and afflictions come from all directions to cover it up.
Letting the gaze run loose also makes the heart blind to distinguishing between truth and falsehood, between the sunnah and innovation; while lowering it for Allah, the Might and Exalted, gives it a penetrating, true and distinguishing insight.
A righteous man once said: "Who'ever enriches his outward behaviour by following the sunnah, and makes his inward soul wealthy through contemplation, and averts his gaze away from looking at what is forbidden, and avoids anything of a doubtful nature, and feeds solely on what is halal - his inner sight will never falter."
Rewards for actions come in kind. Whoever lowers his gaze from what Allah has forbidden, Allah will give his inner sight abundant light.
Too Much Food?
The consumption of small amounts of food guarantees tenderness of the heart, strength of the intellect, humility of the self, weakness of desires, and gentleness of temperament. Immoderate eating brings about the opposite of these praiseworthy qualities.
Al-Miqdam ibn Ma'd Yakrib said: "I heard the Messenger of Allah say:??? "The son of Adam fills no vessel more displeasing to Allah than his stomach. A few morsels should be enough for him to preserve his strength. If he must fill it, then he should allow a third for his food, a third for his drink and leave a third empty for easy breathing."
Excessive eating induces many kinds of harm. It makes the body incline towards disobedience to All?آ¢h and makes worship and obedience seem laborious - such evils are bad enough in themselves. A full stomach and excessive eating have caused many a wrong action and inhibited much worship. Whoever safeguards against the evils of overfilling his stomach has prevented great evil. It is easier for shaytan to control a person who has filled his stomach with food and drink, which is why it has often been said: "Restrict the pathways of shaytan by fasting."
It has been reported that when a group of young men from the Tribe of Israel were worshipping, and it was time for them to break their fast, a man stood up and said:??? "Do not eat too much, otherwise you will drink too much, and then you will end up sleeping too much, and then you will lose too much."
The Prophet and his companions, may Allah be pleased with them, used to go hungry quite frequently. Although this was often due to a shortage of food, Allah decreed the best and most favourable conditions for His Messenger, may Allah bless him and grant him peace. This is why Ibn Umar and his father before him - in spite of the abundance of food available to them - modelled their eating habits on those of the Prophet . It has been reported that Aisha, may Allah be pleased with her, said:??? "From the time of their arrival in Madina up until his death , the family of Muhammad never ate their fill of bread made from wheat three nights in a row."
Ibrahim ibn Adham said:??? "Any one who controls his stomach is in control of his deen, and anyone who controls his hunger is in control of good behaviour. Disobedience towards Allah is nearest to a person who is satiated with a full stomach, and furthest away from a person who is hungry."
Keeping Bad Company?
Unnecessary companionship is a chronic disease that causes much harm. How often have the wrong kind of companionship and intermixing deprived people of Allah's generosity, planting discord in their hearts which even the passage of time-even if it were long enough for mountains to be worn away-has been unable to dispel. In keeping such company one can find the roots of loss, both in this life and in the next life.
A servant should benefit from companionship. In order to do so he should divide people into four categories, and be careful not to get them mixed up, for once one of them is mixed with another, then evil can find its way through to him:
The first category are those people whose company is like food: it is indispensable, night or day. Once a servant has taken his need from it, he leaves it be until he requires it again, and so on. These are the people with knowledge of Allah - of His commands, of the scheming of His enemies, and of the diseases of the heart and their remedies - who wish well for Allah, His Prophet and His servants. Associating with this type of person is an achievement in itself.
The second category are those people whose company is like a medicine. They are only required when a disease sets in. When you are healthy, you have no need of them. However, mixing with them is sometimes necessary for your livelihood, businesses, consultation and the like. Once what you need from them has been fulfilled, mixing with them should be avoided.
The third category are those people whose company is harmful. Mixing with this type of person is like a disease, in all its variety and degrees and strengths and weaknesses. Associating with one or some of them is like an incurable chronic disease. You will never profit either in this life or in the next life if you have them for company, and you will surely lose either one or both of your deen and your livelihood because of them. If their companionship has taken hold of you and is established, then it becomes a fatal, terrifying sickness.
Amongst such people are those who neither speak any good that might benefit you, nor listen closely to you so that they might benefit from you. They do not know their souls and consequently put their selves in their rightful place. If they speak, their words fall on their listeners' hearts like the lashes of a cane, while all the while they are full of admiration for and delight in their own words.
They cause distress to those in their company, while believing that they are the sweet scent of the gathering. If they are silent, they are heavier than a massive millstone-too heavy to carry or even drag across the floor 
All in all, mixing with anyone who is bad for the soul will not last, even if it is unavoidable. It can be one of the most distressing aspects of a servant's life that he is plagued by such person, with whom it may be necessary to associate. In such a relationship, a servant should cling to good behaviour, only presenting him with his outward appearance, while disguising his inner soul, until Allah offers him a way out of his affliction and the means of escape from this situation.
The fourth category are those people whose company is doom itself. It is like taking poison: its victim either finds an antidote or perishes. Many people belong to this category. They are the people of religious innovation and misguidance, those who abandon the sunnah of the Messenger of Allah and advocate other beliefs. They call what is the sunnah a bid'a and vice-versa. A man with any intellect should not sit in their assemblies nor mix with them. The result of doing so will either be the death of his heart or, at the very best, its falling seriously ill.
What Gives the Heart Life and Sustenance
You should know that acts of obedience are essential to the well being of the servant's heart, just in the same way that food and drink are to that of the body. All wrong actions are the same as poisonous foods, and they inevitably harm the heart.
The servant feels the need to worship his Lord, Mighty and Glorious is He, for he is naturally in constant need of His help and assistance.
In order to maintain the well being of his body, the servant carefully follows a strict diet. He habitually and constantly eats good food at regular intervals, and is quick to free his stomach of harmful elements if he happens to eat bad food by mistake.
The well being of the servant's heart, however, is far more important than that of his body, for while the well being of his body enables him to lead a life that is free from illnesses in this world, that of the heart ensures him both a fortunate life in this world and eternal bliss in the next.
In the same way, while the death of the body cuts the servant off from this world, the death of the heart results in everlasting anguish. A righteous man once said, "How odd, that some people mourn for the one whose body has died, but never mourn for the one whose heart has died? and yet the death of the heart is far more serious!"
Thus acts of obedience are indispensable to the well being of the heart. It is worthwhile mentioning the following acts of obedience here, since they are very necessary and essential for the servant's heart: Dhikr of Allah ta'Ala, recitation of the Noble Qur'an, seeking Allah's forgiveness, making du'as, invoking Allah's blessings and peace on the Prophet, may Allah bless him and grant him peace, and praying at night.
 Da`eef hadith, Al-Mundhari, 3/234; and al-Iraqi in al-Ihya, 8/1539.
 Da`eef hadith, at-Tirmidhi, Kitab az-Zuhud, 7/92, gharib; no one else has transmitted it other than Ibrahim ibn Abdullah ibn Hatib, whom ath-Thahabi mentions, 1/43, stating that this is one of the gharib hadith attributed to him.
 Da`eef hadith, Ibn Hibban and al-Baihaqi, and al-Iraqiin his edition of al-Ihya, 8/1541.
 Sahih hadith, at-Tirmidhi, al-Hakim, ath-Thahabi.
 Sahih hadith, at-Tirmidhiand Ahmad; also al-Hakim and ath-Thahabi.
 Al-Bukhari in Kitab ar-Riqaq, and Muslim in Kitab az-Zuhud.
 At-Tirmidhi, Kitab az-Zuhud; he said the hadith is hasan gharib.
 At-Tirmidhi in Kitab az-Zuhud with a slightly different wording; he said the hadith is hasan. This wording is reported by Abu Na`im in al-Hilya.
 Al-Bukhari, Kitab ar-Riqaq, 11/308 and Kitab al-Hudud, 12/113.
 Al-Bukhari, Kitab ar-Riqaq, 11/308; Muslim, Kitab al-Iman, 2/18. The complete hadith is:? "Let whoever believes in Allah and the Last Day either speak good or remain silent; and let whoever believes in Allah and the Last Day be generous to his neighbour; and let whoever believes in Allah and the Last Day be generous to his guest."
 The hadith is hasan and is reported by at-Tirmidhi in Kitab az-Zuhud and by Ibn Majah in Kitab al-Fitan. At-Tirmidhi classifies it as hasan gharib. We have no report of it other than from Muhammad ibn Yazid ibn Khanis.
 Hasan according to Abu Ya`la, Baihaqi and as-Suyuti
 Sahih, at-Tirmidhi, Kitab az-Zuhud, 6/607; Ahmad, al-Musnad, 1/201; as-Sa`ati, al-Fath ar-Rabbani, 19/257; hadith number 12 in an-Nawawi's Forty Hadiths.
 Da`eef, at-Tabarani, 8/63; al-Hakim, al-Mustadrak, 4/314; Ahmad, al-Musnad, 5/264.
 Sahih, Ahmad, al-Musnad, 4/132; as-Sa`ati, al-Fath ar-Rabbani, 17/88; at-Tirmidhi, Kitab az-Zuhud, 7/51.
 Da`eef; it does not appear in most of the sources of the sunnah, but is mentioned in al-Ghazzali's al-Ihya, 8/1488.
 Al-Bukhari, Kitab al-At'ima, 9/549; and Muslim, Kitab az-Zuhud, 8/105.
 Ash-Shafi`ee, may Allah be pleased with him, is reported to have said,??"Whenever a tedious person sits next to me, the side on which he is sitting feels lower down than the other side of me."