"Do you know what backbiting is?" They said: "Allah and His Messenger know best." He said: "It is your mentioning of your brother in a way he would dislike."
The narration continues, in substance: he was asked what if the thing said is true of the man, and he answered that if it is true it is backbiting, and if it is not true it is slander.
Narrated from Abū Hurayrah. Ṣaḥīḥ Muslim 2581 (Kitāb al-Birr).
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In The Tongue
At this point every one of us reaches for the same escape, and the Prophet ﷺ was asked about it directly:
The question in that hadith is our question. Someone put it fourteen centuries ago, in the identical defensive shape we still use, and it was closed where he stood.
Truth is not a defence. Truth is what makes it backbiting rather than slander. Both are sins; the accuracy of your statement determines only which of the two you have committed.
Qur'anic Arabic is reproduced in the Uthmani script. Hadith Arabic is reproduced from the published collections named above. The English is a rendering informed by widely used translations, offered as an aid to meaning and not a substitute for the Arabic. Verify what is cited, and consult people of knowledge on anything affecting practice.