Musannaf Ibn Abi Shayba 21:38

Yazid ibn Harun narrated to us, from Abd al-Malik ibn Abi Sulayman, from Abu al-Zubayr, from Safwan ibn Abdullah, who was married to al-Darda. He came to her and found Umm al-Darda, but did not find Abu al-Darda. She said to him, "Do you intend to perform Hajj this year?" He said, "Yes." She said, "Then supplicate to Allah for us with good, for the Prophet, peace and blessings of Allah be upon him, used to say: 'The supplication of a person for his brother in his absence is answered. At his head is an angel who says "Amen" to his supplication every time he supplicates for him with good, and says: "And for you the same."'" He said: Then I went out to the market and met Abu al-Darda, and he narrated to me from the Prophet, peace and blessings of Allah be upon him, the like of that.