Abu Hurairah (RA) said: The Messenger of Allah (ﷺ) was asked: Messenger of Allah! What is back-biting? He replied: It is saying something about your brother which he would dislike. He was asked again: Tell me how the matter stands if what I say about my brother is true? He replied: If what you say of him is true, you have slandered him, and if what you say of him is not true, you have reviled him.
Sunan Abi Dawud 4874, Book 43, Hadith 102