(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.
Reference: Sahih Muslim 2732 b
In-book reference: Book 48, Hadith 120
USC-MSA web (English) reference(deprecated numbering scheme): Book 35, Hadith 6589