Musnad Ahmad

Hazrat Umm Darda, quoting Abu Darda, narrates that I heard the Prophet say: When a Muslim prays for his brother in his absence, his prayer is accepted, and an angel is appointed at his head for this purpose. Whenever he prays for good for his brother, the angel says "Ameen" to it and says, "And for you the same."

Safwan bin Abdullah, whose wife was Darda, says: Once I came to Syria and visited Hazrat Abu Darda (may Allah be pleased with him), but he was not at home; however, his wife was present. She asked me, "Do you intend to perform Hajj this year?" I replied in the affirmative. She said, "Pray for good for us as well, because the Prophet used to say that when a Muslim prays for his brother in his absence, the prayer is accepted, and an angel is appointed at his head for this purpose. Whenever he prays for good for his brother, the angel says 'Ameen' to it and says, 'May you also have the same.'" Then I went to the market and met Hazrat Abu Darda (may Allah be pleased with him) as well. He also said the same thing to me and narrated the same hadith from the Prophet.