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Al-Adab al-Mufrad 239
حَدَّثَنَا إِبْرَاهِيمُ بْنُ حَمْزَةَ، قَالَ: حَدَّثَنَا ابْنُ أَبِي حَازِمٍ، عَنْ كَثِيرِ بْنِ زَيْدٍ، عَنِ الْوَلِيدِ بْنِ رَبَاحٍ، عَنْ أَبِي هُرَيْرَةَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ: الْمُؤْمِنُ مَرْآةُ أَخِيهِ، وَالْمُؤْمِنُ أَخُو الْمُؤْمِنِ، يَكُفُّ عَلَيْهِ ضَيْعَتَهُ، وَيَحُوطُهُ مِنْ وَرَائِهِ.
Abu Hurayra reported that the Prophet, may Allah bless him and grant him peace, said, "A believer is the mirror of his brother. A believer is the brother of another believer. He protects him against loss and defends him behind his back."
Cited in Al-Adab al-Mufrad (No. 239).