Sahih Muslim | Book 35 | Chapter 22

Narrated by Abu Dharr
Abu Dharr reported that Allah's Messenger (may peace be upon him) 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.

Narrated by Umm Darda
Umm Darda reported: My husband reported that he heard Allah's Messenger (may peace be upon him) 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.

Narrated by Safwan (and he was Ibn 'Abdullah b. Safwan, and he had been married to Umm Darda')
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 him) 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 (may peace be upon him).

Narrated by Safwan b. 'Abdullah b. Safwan
This hadith has been narrated on the authority of Safwan b. 'Abdullah b. Safwan with the same chain of transmitters.