Gheebah (backbiting, gossip) means that a person mentions the faults of his Muslim brother in his absence, which he would not like if he heard about it, when there is no need to mention them.
When I say “mentions the faults of his brother”, this excludes cases when the other person says something to praise or commend him.
When I say “Muslim brother”, this excludes the kafir (non-Muslim), for there is no gheebah in the case of a kafir.
When I say, “in his absence”, this excludes things said in his presence, which is not called gheebah according to the more correct of the two scholarly opinions.
When I say, “which he would dislike if he heard about it”, this excludes things which he would not mind.
When I say, “when there is no need to mention them”, this excludes cases when there is a shar’i (legal religious) reason for doing that, such as warning against an innovator to make people aware of his bid’ah (innovation).
It is essential to pay attention to the following in such cases:
1. Sincerity towards Allah and seeking His pleasure.
2. Paying attention to the interests being served by such things.
3. What is said should be limited to the shortcomings in question and should not go further, to matters in which there is no benefit.
The scholars agreed that it is haram (impermissible) to talk behind a person’s back for no legitimate purpose. Most of them stated that this is a major sin and that it varies in degree, some kinds being worse than others. The one who backbites about a scholar is not like one who backbites about an ignorant person. Allah says (interpretation of the meaning):
“… neither backbite one another. Would one of you like to eat the flesh of his dead brother? You would hate it (so hate backbiting). And fear Allah. Verily, Allah is the One Who forgives and accepts repentance, Most Merciful” [49:12]
In Saheeh Muslim it is narrated from al-‘Ala ibn ‘Abd al-Rahman from his father from Abu Hurayrah (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) said: “Do you know what gheebah is?” They said, “Allah and His Messenger know best.” He said, “That you say something about your brother that he dislikes.” He was asked, “What if what I say about my brother is true?” He said, “If what you say is true then you have gossiped about him, and if it is not true then you have slandered him.”
One of the worst types of gheebah and one which is most emphatically forbidden is to look down upon a Muslim and do one’s utmost to insult him, show disrespect towards him and cast aspersions upon his honour. This is a blameworthy characteristic and a serious malady; it is one of the major sins and the one who does this is subject to the warning and a severe punishment.
And Allah knows best.
Source : Islamqa