Umm Darda' reported: My husband reported that he heard Allah's Messenger (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.
Hadith Referenceصحيح مسلم / كتاب الذكر والدعاء والتوبة والاستغفار / 2732