كتاب الذكر والدعاء والتوبة والاستغفار
48
The Book Pertaining to the Remembrance of Allah, Supplication, Repentance and Seeking Forgiveness
(23)
Chapter: The Virtue Of Praying For The Muslims In Their Absence
(23)
باب فَضْلِ الدُّعَاءِ لِلْمُسْلِمِينَ بِظَهْرِ الْغَيْبِ ‏‏

Umm Darda' reported:

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.
حَدَّثَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ، أَخْبَرَنَا النَّضْرُ بْنُ شُمَيْلٍ، حَدَّثَنَا مُوسَى بْنُ سَرْوَانَ، الْمُعَلِّمُ حَدَّثَنِي طَلْحَةُ بْنُ عُبَيْدِ اللَّهِ بْنِ كَرِيزٍ، قَالَ حَدَّثَتْنِي أُمُّ الدَّرْدَاءِ، قَالَتْ حَدَّثَنِي سَيِّدِي، أَنَّهُ سَمِعَ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ ‏ "‏ مَنْ دَعَا لأَخِيهِ بِظَهْرِ الْغَيْبِ قَالَ الْمَلَكُ الْمُوَكَّلُ بِهِ آمِينَ وَلَكَ بِمِثْلٍ ‏"‏ ‏.‏
Reference : Sahih Muslim 2732 b
In-book reference : Book 48, Hadith 120
USC-MSA web (English) reference : Book 35, Hadith 6589
  (deprecated numbering scheme)