Sunan Abi Dawud

Abu Hurairah 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