Sahih Muslim Book 35, Hadith Number 6590.
Chapter : Excellence of supplicating for the Muslims behind their back (in their absence).
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).