Al-Adab Al-Mufrad, Book 12, Hadith 239 (English) ⓘ

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."

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.‏