Hadith 1523

Abu Hurairah (May Allah be pleased with him) said: The Messenger of Allah (PBUH) said, "Do you know what is backbiting?" The Companions said: "Allah and His Messenger know better." Thereupon he said, "Backbiting is talking about your (Muslim) brother in a manner which he dislikes." It was said to him: "What if my (Muslim) brother is as I say." He said, "If he is actually as you say, then that is backbiting; but if that is not in him, that is slandering."

[Muslim].