Sunan At-Tirmidhi

Abu Hurairah narrated that it was said:"" O Messenger of Allah! What is backbiting?"" He said : "" Mentioning your brother with that which he does not like."" He said "" What if what I said about him is so?"" He said: ""If what you said about him is so, then you have backbitten him, and if it is not as you said, then you have slandered him