Prophet Muhammad informed us in a very famous narration, commonly quoted with reference to lawful and unlawful, about the important role of the heart. He said: “Both lawful and unlawful matters are evident but in between them there are doubtful (suspicious) things and many people have no knowledge about them. So whoever saves himself from these suspicious matters (by staying away from them) saves his religion and his honor. And whoever indulges in these suspicious matters (will eventually) indulge in what is forbidden. This is like (the example of) a shepherd who grazes (his animals) near the Hima (private pasture) of someone else and at any moment he is liable to get in it. (O people!) Beware! Every king has a Hima and the Hima of Allah on the earth is His illegal (forbidden) matters. Beware! There is a piece of flesh in the body, if it is reformed, the whole body becomes good, but if it gets spoilt the whole body gets spoilt; indeed it is the heart.” [Al-Bukhari]
He said that after explaining that the lawful matters are clear and that the forbidden ones are clear and that between them are obscure areas, not known to many people. However, what protects a person from the forbidden matters and ensures that he remains in the lawful ones is knowledge; but beyond knowledge, it is the state of the heart. If the heart is good, then it makes use of the knowledge and it avoids what is prohibited. If the heart is corrupted, then the knowledge is of no benefit to it and it will indulge in what is prohibited.
The Prophet during the last (and only) pilgrimage, informed his Companions and it was an instruction to the entire Muslim nation to come, that people are not favored based on their race or color, but rather based on piety and on how much they are Allah fearful. After that, he clarified that the place of piety is in one’s heart.”
In these statements and other similar statements, we find stress being placed on the heart - that the heart is the part of the body, which Allah has favored over all other parts. It is the place of faith, and had there been in the body another part that was nearer to Allah, piety would have been placed there, because faith is the most valuable thing that a human possesses. It (faith) is the determination ultimately of those who have belief in Allah - those who have accepted the message and who have chosen Paradise over Hell. It is the distinction of those who have belief and those who have disbelief.
The value of faith is greater than the value of all the things of this world. This is why Prophet Muhammad said that for Allah to guide by your hands a single person to Islam is worth more than anything in this world. For you to help someone to find faith is worth more than any of the things in this world. [Al-Bukhari]
The heart is the place in which the correctness of deeds is judged. The Prophet said: "Deeds are judged according to the intention." The place of the intention is not on the lips. It is in the heart.
'Umar Ibn Al-Khattaab narrated that the Messenger of Allah said: "The reward of deeds depends upon the intention and every person will get the reward according to what he has intended…” [Al-Bukhari]
Our deeds - what we do externally - are judged ultimately by the states of our hearts. Good deeds are in reference to those that we perceive to be a part and parcel of righteousness. Allah will inspect the hearts to determine whether they are truly acts of righteousness.
The Prophet informed us that the first three people who would be cast into the Hell Fire are those who were involved in what everybody considers great acts of righteousness. They are the scholar who taught knowledge; the wealthy person who gave from his wealth in charity and the martyr who gave his life fighting in the path of Allah.
The Prophet in an authentic narration, said that some people from among these types of people would be among the first groups of people thrown into Hell because the scholar, when he taught the knowledge that Allah gave him, did not do so for the sake of Allah. He taught so that people would praise him, saying what a great scholar he was and how knowledgeable he was. Allah will Say to him: "You received your praise, what you sought in that world. But there will be nothing for you in the Next." So he will be drawn off on his face and thrown into Hell.
Similarly, the benefactor, who was generous with his wealth. He gave and people praised his generosity, but Allah will Say: "You did it for the praise and you were praised. You did it for people to appreciate you." So that individual will be drawn off on his face and thrown into Hell. [Muslim]
The martyr - the one whom we all assumed had died in the Path of Allah. We would think that his place in Paradise is guaranteed. But Allah will Say: "You fought so people would say, 'How brave this one is! How strong and courageous he was!'" People said it; they praised you, so he will be drawn off on his face and thrown into Hell.’
All is telling us that ultimately, even the highest of deeds can be of no avail if the hearts are sick; if the hearts are corrupt. So the place of the heart should, in our minds, occupy great attention. We have to spend much of our time observing, being aware of the state of our heart.
So there is no other faculty in the human body and existence that a believer should be more concerned about. We have to make sure that this faculty is functioning as Allah wishes it to function. We should be greatly concerned about it. The Prophet often used to supplicate, beginning with: "I seek refuge in You, O Allah, from knowledge that does not benefit, and from a heart which does not fear." [At-Tirmithi, Abu Daawood, An-Nasaa’i and Ibn Maajah]