Abu Dharr 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 not say: The same be for you too.
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.
Safwan (and he was Ibn 'Abdullah b. 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 (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 supplica- tion for blessings for his brother and the commissioned Angel says: Amen, and says: May it be for you too I I went to the bazar and met Abfi Dardi' and he narrated like this from Allah's Messenger (ﷺ).
This hadith has been narrated on the authority of Safwan b. 'Abdullah b. Safwan with the same chain of transmitters.