Bulugh al-Maram, Book 16, Hadith 1495 (In-Book)

Abu Hurairah (RAA) narrated, ‘The Messenger of Allah asked, “Do you know what backbiting is?” They replied, `Allah and His Messenger 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 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.

وَعَنْ أَبِي هُرَيْرَةَ ‏- رضى الله عنه ‏- أَنَّ رَسُولَ اَللَّهِ ‏- صلى الله عليه وسلم ‏-قَالَ: { أَتَدْرُونَ مَا اَلْغِيبَةُ?

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also referenced under: Book 16, Hadith 1538 (English)

قَالُوا: اَللَّهُ وَرَسُولُهُ أَعْلَمُ.‏

قَالَ: ذِكْرُكَ أَخَاكَ بِمَا يَكْرَهُ.‏

قِيلَ: أَرَأَيْتَ إِنْ كَانَ فِي أَخِي مَا أَقُولُ?

قَالَ: إِنْ كَانَ فِيهِ مَا تَقُولُ فَقَدْ اِغْتَبْتَهُ, وَإِنْ لَمْ يَكُنْ فَقَدْ بَهَتَّهُ } أَخْرَجَهُ مُسْلِمٌ.‏ (1958)‏.‏ ‏5 ‏- صحيح.‏ رواه مسلم (2589)‏.‏