The similitude of believers in regard to mutual love, affection, fellow-feeling is that of one body; when any limb of it aches, the whole body aches, because of sleeplessness and fever. (Using translation from Muslim 2586a)   

عَنِ النَّبِيِّ ﷺ قَالَ مَثَلُ الْمُؤْمِنِينَ فِي تَوَادِّهِمْ وَتَعَاطُفِهِمْ وَتَرَاحُمِهِمْ مَثَلُ الْجَسَدِ إِذَا اشْتَكَى مِنْهُ عُضْوٌ تَدَاعَى سَائِرُ الْجَسَدِ بِالسَّهَرِ وَالْحُمَّى أَخْرَجَاهُ فِي الصَّحِيحِ مِنْ حَدِيثِ زَكَرِيَّا  

My husband reported that he heard Allah's Mes- senger ﷺ as saying: He who supplicates for his brother behind his back (in his absence), the Angel commissioned (for carrying supplication to his Lord) says: Amen, and it is for you also. (Using translation from Muslim 2732b)   

أَنَّهُ سَمَعَ رَسُولَ اللهِ ﷺ يَقُولُ مَنْ دَعَا لِأَخِيهِ بِظَهْرِ الْغَيْبِ قَالَ الْمَلَكُ الْمُوَكَّلُ بِهِ آمِينَ وَلَكَ بِمِثْلٍ