Hadith: Do you know what backbiting is? They said: Allah and His Messenger know best. He said: It is to say of your brother what he dislikes

عن أبي هريرة رضي الله عنه عن النبي صلى الله عليه وسلم قال: «أتدرون ما الغِيبَةُ؟»، قالوا: الله ورسوله أعلم، قال: «ذكرُك أخاك بما يكره»، قيل: أرأيت إن كان في أخي ما أقول؟ قال: «إن كان فيه ما تقول فقد اغْتَبْتَهُ، وإن لم يكن فقد بَهَتَّهُ».
[صحيح] - [رواه مسلم]

Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "c2">“Do you know what backbiting is?” They (the Companions) said: "c2">“Allah and His Messenger know best.” He said: "c2">“It is to say of your brother what he dislikes.” It was said: "c2">“What do you think if what I say of my brother is true?” He said: "c2">“If what you say of him is true, then you have backbitten him; if not, then you have slandered him.”
Sahih/Authentic. - [Muslim]

Explanation

The Prophet (may Allah's peace and blessings be upon him) clarified backbiting. It means that a Muslim speaks unfavorably of another Muslim who is absent in the sense that he would find faults with his brother’s manners or physique. If the indicated fault really exists, this is backbiting. If it does not exist, it constitutes slander.