The Heart:
The most important part of the body is the heart as has been mentioned by our noble Prophet Muhammed (SAW) in his saying, “Indeed there is a piece of flesh in your body that, if it is sound, then the whole body will be sound and if it is corrupt then the whole body will be corrupt. Indeed it is the heart.” – Sahih al-Bukhari & Sahih al-Muslim
The heart is thus the port where all actions stem, whether good or evil. If the heart is good then the actions will be good and if the heart is corrupt then the actions will be bad. The heart is the place where Allah scans, as the Messenger of Allah (SAW) says, “Allah does not look at your bodies nor your faces but he scans your heart and actions.” – Sahih al-Muslim
As for the heart, Allah has commanded us to love Him and His Messenger, to love the righteous people, to love that which Allah loves, and to hate all which Allah hates. The tongue acts accordingly, because…. the tongue expresses what is in the heart.
The Tongue Expresses what is in the Heart:
If a person is good, it reflects in their saying. Allah has made it very easy for the tongue to move, and it is not difficult to realize that a person’s tongue moves much more compared to the limbs. So if a person is not careful about what he says, even a single saying of his can invalidate his actions. It is for this reason that the whole body warns the tongue saying, “Fear Allah concerning us, for we are (dependant) upon you. If you are upright then we will be upright and if you are corrupt then we will be corrupt.” – Sunan at-Tirmidhi, Mishkaat, Riyaadh as-Saleheen
And at the same time, the tongue is a great blessing from Allah. If we look at the animals, who are unable to speak, we realize the importance of this great tool of communication.
The gratitude that we should show for this great blessing should include, using it for obedience to Allah, for reading the Qur’an, for seeking Knowledge, to enjoin good, and forbidding evil. The prohibitions of the tongue are that you do not lie, abuse, use foul language, backbite and slander.
Allah says at the beginning of Soorah al-Mumineen, “Indeed successful are the Believers, those who in their prayer have Khushoo [humility/fear of Allah]” (fear of Allah) and those who refrain from vain talk.” [Soorah Mumineen : 1-3]
Here Allah mentions that refraining from vain talk is a sign of successful believers. Allah also mentions refraining from vain talk after he mentions al-Khushoo. This is because too much talk makes the heart hard. It is not possible to reach the level of Khushoo unless one refrain from vain talk.
Imaam an-Nawawee (RA) mentions “Know it is incumbent upon all to guard their tongues from most speech, except a word that has some benefit in it….” – Kitaabul-Adhkaar
And for those who take heed, there is a constant reminder in the verse, “Not a word is said except that there is a watcher by him ready to record it.” – [Soorah Qaf (50):18]
The Messenger of Allah, Muhammed (SAW) said, “Whoever believes in Allah and the Last Day should speak good or keep silent.”