Abu Hurairah (May Allah be pleased with him) said:
The Messenger of Allah (ﷺ) 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].
وعن أبي هريرة رضي الله عنه أن رسول الله صلى الله عليه وسلم 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 ما تقول فقد بهته”. ((رواه مسلم)).
Sunnah.com reference : Book 18, Hadith 13
Arabic/English book reference : Book 18, Hadith 1523
http://sunnah.com/riyadussaliheen/18/13
Abu Huraira reported Allah's Messenger (may peace be upon him) as saying: Avoid suspicion, for suspicion is the gravest lie in talk and
do not be inquisitive about one another and do not spy upon one another and do not feel envy with the other, and nurse no malice, and nurse no aversion and hostility against one another. And be fellow-brothers and servants of Allah.
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