The Messenger of Allah ﷺ said, "There is no Muslim servant who prays for his brother in his absence except that the angel says, 'And the same for you.'"
قال رسول الله ﷺ: ما من عبد مسلم يدعو لأخيه بظهر الغيب إلا قال الملك: ولك بمثل.
Muslim narrated it in the Book of Remembrance, Supplication, Repentance, and Seeking Forgiveness (2732), from Ahmad bin Umar bin Hafs Wakii, who said, Muhammad bin Fudayl narrated to us, from his father, from Talha bin Ubaydullah bin Kurayz, from Umm Darda, from Abu Darda, who said, and then he mentioned it.His words, "and seek forgiveness for those who believe" [Ghafir 40:7], mean that the angels pray for the believers and seek forgiveness for them. Part of this is that they say "Amen" to the supplication a believer makes for his fellow believer in his absence.