Bulugh al-Maram, Book 16, Hadith 1528 (In-Book)

Abu ad-Darda’ (RAA) narrated that the Messenger of Allah said: “If a Muslim defends his brother’s honor in his absence, Allah will protect his face from the fire of Hell on the Day of Resurrection.” Related by At-Tirmidhi who graded it to be Hasan.

وَعَنْ أَبِي اَلدَّرْدَاءِ ‏- رضى الله عنه ‏- عَنْ اَلنَّبِيِّ ‏- صلى الله عليه وسلم ‏-قَالَ: { مَنْ رَدَّ عَنْ عِرْضِ أَخِيهِ بِالْغَيْبِ, رَدَّ اَللَّهُ عَنْ وَجْهِهِ اَلنَّارَ يَوْمَ اَلْقِيَامَةِ } أَخْرَجَهُ اَلتِّرْمِذِيُّ, وَحَسَّنَهُ.‏ (1990)‏.‏ ‏3 ‏- حسن.‏ رواه الترمذي (1931)‏، وقال:" هذا حديث حسن".‏ قلت: ويشهد له ما بعده.‏

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also referenced under: Book 16, Hadith 1571 (English)