The Messenger of Allah ﷺ asked, "Do you know what backbiting is?"They replied, "Allah and His Messenger know best."He said, "It is mentioning your brother in a way he would dislike."Someone asked, "What if what I say about my brother is actually true?"He answered, "If what you say is true, then you have backbitten him, and if it is not true, then you have slandered him."
أن رسول الله ﷺ قال: أتدرون ما الغيبة؟ قالوا: الله ورسوله أعلم. قال: ذكرك أخاك بما يكره قيل: أفرأيت إن كان في أخي ما أقول؟ قال: إن كان فيه ما تقول فقد اغتبته، وإن لم يكن فيه فقد بهته.
Muslim narrated it in the Book of Righteousness, Maintaining Ties of Kinship, and Manners (2589), through several chains from Ismail bin Jafar, from Ala, from his father, from Abu Hurairah, who mentioned it.