Abu Hurairah (RAA) narrated, 'The Messenger of Allah (ﷺ) asked, "Do you know what backbiting is?" They replied, `Allah and His Messenger (ﷺ) know best.' He said, "It is saying something about your brother which he dislikes." Someone asked, 'Supposing that what I said about my brother was true?' and the Messenger of Allah (ﷺ) said:
"If what you say about him is true you have backbitten him and if it is not true you have slandered him." Related by Muslim.
English reference: Book 16, Hadith 1538
Arabic reference: Book 16, Hadith 1495