It is narrated from Safwan bin Abdullah bin Safwan (may Allah have mercy on him) — and Umm al-Darda (may Allah be pleased with her) loved him — he said: I went to their house and found Umm al-Darda (may Allah be pleased with her), but did not find Sayyiduna Abu al-Darda (may Allah be pleased with him). Umm al-Darda (may Allah be pleased with her) said to me: "Do you intend to perform Hajj this year?" I said: "Yes." She said: "Pray to Allah for good for us, for the Prophet (peace and blessings be upon him) used to say: 'The supplication of the wronged person for his brother in his absence is answered. At his head there is an angel who keeps saying Amin to his supplication; whenever he prays for good for his brother, that angel says: Amin, and for you the like of it.'" Safwan said: Then I went out to the market and met Abu al-Darda (may Allah be pleased with him), and he too narrated to me from the Prophet (peace and blessings be upon him) a hadith like it.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 21021