Islam prohibits backbiting

Backbiting and slandering are shame and disgrace. Their perpetrator is
detested and he shall not have a noble death. Allah forbids these acts, as He says in the Qur'an:

"Backbiting and Gossiping are from the most vilest and despicable of
things, yet the most widely spread amongst mankind, such that no one is free from it except for a few people."

Backbiting (gheebah) means mentioning something about a person (in his absence), that he hates (to have mentioned), whether it is about: His body, his religious characteristics, his worldly affairs, his self, his physical appearance, his character, his wealth, his child, his father, his wife, his manner of walking, his smile, it is the same whether you mention that about him with words, through writings, or whether you point or indicate him by gesturing with your eyes, hand or head.

As for the body, is when you make fun of how someone looks, or mentioning any bad quality in him, as saying: "he is blind", "he limps", "he is bleary-eyed", "he is bald", "he is short", "he is tall", "he is black", "he is yellow", "he's too thin", "he's too fat". As for his religious qualities, it is when you say: "he is a sinner", "he is a thief", "he is a betrayer", "he is an oppressor", "he doesn't pray", "he prays so fast", "he does not behave well towards his parents", "he does not pay the Zakat duly"." As for the worldly matters, then it is when you say: "he has poor manners", "he does not think that anyone has a right over him", "he talks too much" …etc

Allah says in the Qur'an:

"O you who believe! Avoid much suspicion, in deeds some suspicions
are sins. And spy not 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 accepts repentance, Most Merciful"
(Qur'an 49: 12)

In this verse, Allah strongly forbids backbiting, and he compares the backbiter to one who eats the flesh of his dead brother. If he would hate eating the flesh of his brother, he should also hate to eat his flesh while he is alive by backbiting and slandering him.

When one reflects deeply over this assimilation it will be enough to keep one away from backbiting.

Abu Hurayrah (May Allah be pleased with him) narrated that Prophet Mohammad (PBUH) said: "Do you know what backbiting is?” They said, “Allah and His Messenger know best.” He then said, “It is to say something about your brother that he would dislike.” Someone asked him, “But what if what I say is true?” The Messenger of Allah said, “If what you say about him is true, you are backbiting him, but if it is not true then you have slandered him."

Backbiting is so widespread among people that it has become an issue of people’s meetings and an avenue for expressing their anger, misgivings and jealousy. Those who indulge in backbiting are hiding their own imperfections and harming others. They are oblivious of the fact that they are only harming themselves.

This is because the backbiter if the wrongdoer and his victim is the wronged, on the Day of Resurrection both the wrongdoer and the wronged will stand before Allah Who is the Just Judge and the wronged will appeal to Allah to avenge the wrong done to him, Allah will then give this wronged person from the good deeds of the person who wronged him in accordance with his wrong by backbiting his brother on a Day that no father will give his son any of his good deeds nor a friend to his friend. All will be saying, ‘Myself, myself.’

Islam prohibits backbiting and slandering - from Islam Online