The Prophet Muhammad (may Allah exalt his mention), informed us in a very famous Hadeeth (narration), commonly quoted with reference to what is lawful and what is unlawful, about the important role of the heart. He said: ““What is lawful is clear and what is forbidden is clear, and in between them there are some matters that are doubtful, which most people do not know –if they are lawful or forbidden. Thus, whoever avoids what is doubtful, safeguards his religion and honor, but one who engages in what is doubtful, falls in what is forbidden. The example of this is like the shepherd who grazes his herd in the vicinity of an exclusively reserved pasture but it is always apprehensive that some of his animals might get into the pasture. Every king has a reserved pasture and the reserved pasture of Allah is what He has forbidden. 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 as well; indeed it is the heart.”.” [Al-Bukhari]

He said this after explaining that the lawful matters are clear and that the forbidden ones are clear, and that between them are obscure matters, not known to many people. However, what protects a person from the forbidden matters and ensures that he remains in 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, may Allah be pleased with them, 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 fearing. 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 closer to Allah, piety would have been placed there, because faith is the most valuable thing that a human being 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 a single person to Islam thanks to you, it is better than anything in this world. For you to help someone to find faith, it is better than everything that is 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, may Allah be pleased with him, 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 state 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 Hellfire 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 Hadeeth, 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 his knowledge 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 same thing will happen to the benefactor who was generous with his wealth. He donated in charity 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 as well be drawn off on his face and thrown into Hell. [Muslim]

Also, we would think that the place of the martyr - whom we all assumed had died in the Path of Allah –in Paradise is guaranteed. But Allah will Say to him: "You fought so that 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 this 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 the knowledge that does not benefit, and from a heart which does not fear." [At-Tirmithi, Abu Daawood, An-Nasaa’i and Ibn Maajah]