Sunnah.com reference : Book 16, Hadith 59English translation : Book 16, Hadith 1538Arabic reference : Book 16, Hadith 1495
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.
وَعَنْ أَبِي هُرَيْرَةَ ‏- رضى الله عنه ‏- أَنَّ رَسُولَ اَللَّهِ ‏- صلى الله عليه وسلم ‏-قَالَ: { أَتَدْرُونَ مَا اَلْغِيبَةُ? 

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.‏  1‏ .‏‏1 ‏- صحيح.‏ رواه مسلم (2589)‏.‏