Rasulullah Sallallahu Alayhi Wasallam said defined Gheebat (Backbiting) when he said: "Do you know what is meant by backbiting?" They said, "Allah and His Messenger know best." Rasulullah Sallallahu Alayhi Wasallam said, "To say something about your brother which he dislikes." One asked, "Even if what I say is true about my brother?" Rasulullah Sallallahu Alayhi Wasallam 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 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 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.
The tongue alone does not do backbiting; it can also be done with the eyes, hands and other movements. For example, imitating somebody who is limping, in order to insult him/her.
Rasulullah Sallallahu Alayhi Wasallam said:
"Backbiting is a worse sin than adultery." It is further explained in a saying of Rasulullah Sallallahu Alayhi Wasallam: "Allah the Exalted may forgive a person if he/she repents after committing adultery. However, Allah the Exalted will not forgive the one who backbites, till his/her victim forgives him/herwcs
