(12) Cheerfulness Towards People
كتاب الِانْبِسَاطِ إِلَى النَّاسِ
(120) Chapter: A Muslim is the mirror of his brother
(120) بَابُ الْمُسْلِمُ مَرْآةُ أَخِيهِ
Al-Adab Al-Mufrad 239 
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.‏
Grade :  Hasan (Al-Albani) حـسـن  (الألباني)حكم :
Reference : Al-Adab Al-Mufrad 239
In-book reference : Book 12, Hadith 2
English translation : Book 12, Hadith 239