The Prophet ﷺ said, "The supplication of a Muslim for his brother in his absence is answered. At his head is an angel appointed over him, and whenever he supplicates for his brother with good, the angel appointed over him says, 'Amen, and may you have the same.'"
كان النبي ﷺ يقول: دعوة المرء المسلم لأخيه بظهر الغيب مستجابة، عند رأسه ملك موكل، كلما دعا لأخيه بخير، قال الملك الموكل به: آمين ولك بمثل.
Muslim narrated it in the Book of Remembrance and Supplication (2733), from Ishaq bin Ibrahim, [who said] Isa bin Yunus informed us, [who said] Abdul-Malik bin Abu Sulayman narrated to us, from Abu Zubayr, from Safwan (who is the son of Abdullah bin Safwan, and Umm Darda was married to him). He said, "I came to Syria and went to Abu Darda's house, but I did not find him there. I found Umm Darda, and she said, 'Do you intend to perform Hajj this year?' I said, 'Yes.' She said, 'Then pray for us for good, for the Prophet ﷺ said,' and she mentioned it. He said, 'So I went out to the market and met Abu Darda, and he told me the same thing,' narrating it from the Prophet ﷺ."Muslim also narrated it through another route from Abdul-Malik bin Abu Sulayman, with the same wording.