Bismillahir Rahmanir Rahim
Alhamdulillah.

Abu Al-Darda Radi Allahu Anhu se rivayat hai ki RasoollAllah Sallallahu Alaihi Wa sallam ne farmaya Jab ek Musalman apney Bhai ke liye gaybana dua karta hai to farishtey aameen kahtey hain aur kahtey hain ki tujhe bhi ye naseeb ho.

[Sunan Abu Dawud, Vol 1 , 1521-Sahih]

English Translation of Hadith:
Abu Al-Darda Radi Allahu Anhu said I heard the Messenger of Allah Sallallahu Alaihi Wa sallam say: When a Muslim supplicates for his absent brother the angels say: Amin, and may you receive the like.

[Sunan Abu Dawud, book 8, 1529-Sahih]