Sunan Ibn Majah, Chapter: The Chapters on Hajj

said that he was married to a daughter of Abu Darda. He came to her and found Umm Darda there, but he did not find Abu Darda. She said to him: "Do you intend to perform Hajj this year?" He said: "Yes." She said: "Pray to Allah for us to grant us goodness, for the Prophet (saw) used to say: 'The supplication of a man for his brother in his absence will be answered. By his head there is an angel who says Amin to his supplication, and every time he prays for his brother, he says: "Amin, and the same for you.'" He said: "Then I went out to the marketplace where I met Abu Darda', and he told me something similar from the Prophet (saw)."