Sahih Muslim #6927 · The Book Pertaining to the Remembrance of Allah, Supplication, Repentance and Seeking Forgiveness

Sahih MuslimAbu Dharr reported that Allah's Messenger
Sahih Muslim · Book 48 · The Book Pertaining to the Remembrance of Allah, Supplication, Repentance and Seeking Forgiveness
Sahih Muslim · كتاب 48 · كتاب الذكر والدعاء والتوبة والاستغفار
There is no believing servant who supplicates for his brother behind his back (in his absence) that the Angels do not say: The same be for you too.
حَدَّثَنِي أَحْمَدُ بْنُ عُمَرَ بْنِ حَفْصٍ الْوَكِيعِيُّ، حَدَّثَنَا مُحَمَّدُ بْنُ فُضَيْلٍ، حَدَّثَنَا أَبِي، عَنْ طَلْحَةَ بْنِ عُبَيْدِ اللَّهِ بْنِ كَرِيزٍ، عَنْ أُمِّ الدَّرْدَاءِ، عَنْ أَبِي الدَّرْدَاءِ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏
"‏ مَا مِنْ عَبْدٍ مُسْلِمٍ يَدْعُو لأَخِيهِ بِظَهْرِ الْغَيْبِ إِلاَّ قَالَ الْمَلَكُ وَلَكَ بِمِثْلٍ ‏"‏ ‏.‏
Reference : Sahih Muslim 6927In-book reference : Book 48, Hadith 119