Abu Dharr (RA) reported that Allah's Messenger (ﷺ) said: There is no believing servant who supplicates for his brother behind his back (in his absence) that the Angels do say: The same be for you too.
Hadith Referenceصحيح مسلم / كتاب الذكر والدعاء والتوبة والاستغفار / 2732
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
Safwan (and he was Ibn 'Abdullah bin Safwan, and he had been married to Umm Darda') reported: I visited Abu Darda's house in Syria. I did not find him there but Umm Darda' (was present at the house). She said: Do you intend to perform Hajj during this year? I said: Yes. She said: Do supplicate Allah for blessings upon us, for Allah's Apostle (ﷺ) (may peace be upon hiin) used to say: The supplication of a Muslim for his brother at his back (in his absence) is responded so long as he makes a supplication for blessings for his brother and the commissioned Angel says: Amen, and says: May it be for you too. I went to the bazar and met Abfi Dardi' and he narrated like this from Allah's Messenger (ﷺ) .
Hadith Referenceصحيح مسلم / كتاب الذكر والدعاء والتوبة والاستغفار / 2733