Allah (SWT) the Exalted said in Noble Qur'an: "O' ye who believe! Avoid suspicion as much (as possible): for suspicion in some cases is a sin: And spy not on each other behind their backs. Would any of you like to eat the flesh of his dead brother? Nay, ye would abhor it. But fear Allah: For Allah is Oft-Returning, Most Merciful." [Sura Hujurat 49:12]
Gossiping will also corrupt the tongue. The act of gossiping is bad since both the speaker and the listener are attacking a defenseless person. It is as if one eats the flesh of his dead defenseless brother and the other one attacks one who is not present there to defend himself. Four points are stressed in the Noble Qur'an in this regard:
(1) - Muslims and brothers in faith are just like our brothers.
(2) - A Muslim's honor is just like his flesh.
(3) - Gossiping behind his back to hurt his honor is like eating his flesh.
(4) - The fact that he is absent and is unable to defend himself is similar to him being dead. It is not right to attack a dead defenseless person.
That is why Imam Ali (as) has said: "Gossiping is done by weak people."
The second verse from Noble Qur'an regarding this issue is: "Those who love (to see) scandal published broadcast among the Believers, will have a grievous Penalty in this life and in the Hereafter: Allah knows, and ye know not." [Sura Al-Noor 24:19]
Man is a social creature and the society in which he lives is like his house. He should safeguard it just as he safeguards his house and prevent the influence of any corruption into the society. Islam seriously fights whatever corrupts the society. Gossiping is also seriously fought with because it unveils hidden flaws. Islam does not like this and instructs people to cover up each other's faults.
The third verse from Noble Qur'an regarding this issue is: "Allah loveth not that evil should be noised abroad in public speech, except where injustice hath been done; for Allah is He who heareth and knoweth all things." [Sura An-Nisa 4:148]
We must realize that any form of evil in public speech has been forbidden here. Thus gossiping is forbidden.
The fourth verse from Noble Qur'an in this regard is as follows: "Woe to every (kind of) scandal-monger and backbiter." [Sura Al-Humaza 104:1]
Some commentators have said that this verse was revealed about al-Walid ibn al-Mughirah who used to gossip behind Prophet Muhammad's (saw) back and made fun of him up front.
16 - Gheebat, Gheebah (Backbiting):
Prophet Muhammad (saw) said: "Do you know what is meant by Gheebat (Backbiting)?" Companions said, "Allah (SWT) and His Messenger know best." Prophet Muhammad (saw) said, "To say something about your brother which he dislikes." One asked, "Even if what I say is true about my brother?" Prophet Muhammad (saw) replied, "If such defects you say are true about him, then you have backbitten him, and if he doesn't have what you say, then you have committed Tohmat, Buhtan (slander) against him."
Hence, it is not allowed to talk about somebody in his/her absence, even if what was narrated was true. If it was not true, then it would be a bigger sin and it is called Buhtan (false accusation).
Allah (SWT) the Exalted made it clear in Noble Qur'an that defaming another Muslim in his/her absence is like eating the flesh of one's dead brother (49:12), which obviously, everybody hates. Hence, it is the most dreadful sin. Note that if the person is present, he/she may have a chance to defend himself/herself, although everybody does not have the courage to defend themselves in these circumstances. If, however, he/she is defamed in his/her absence, the damage is deep and somewhat permanent.
Not just only can the tongue be used to backbite. The same effects and results can also be done with the eyes, hands and other physical movements. An example would be to imitate someone who is limping in order to insult him/her.
Prophet Muhammad (saw) said: "Backbiting is a worse sin than adultery." It is further explained in a saying of Prophet Muhammad (saw): "Allah (SWT) the Exalted may forgive a person if he/she repents after committing adultery. However, Allah (SWT) the Exalted will not forgive the one who backbites, till his/her victim forgives him/her."
"If people talk behind your back - what does that mean? It simply means that you are two steps ahead of them."
Effects and Results of Controlling, Taming and Corruption of the Human Tongue in Islam