He was married to Darda. He said, "I came to Sham and went to visit Abu Darda but did not find him. I found Umm Darda instead, and she asked, 'Do you intend to perform Hajj this year?'I replied, 'Yes.'She said, 'Then pray for good on our behalf, for the Prophet ﷺ used to say, "The supplication of a Muslim for his brother in his absence is answered. At his head is an angel appointed to him, and whenever he prays for his brother with good, the angel says, Ameen, and may you have the like."'Then I went out to the market and met Abu Darda, and he said the same thing to me, narrating it from the Prophet ﷺ."
حدثنا ابن نمير حدثنا عبد الملك عن عطاء
وكانت تحته الدرداء قال أتيت الشام فدخلت على أبي الدرداء فلم أجده ووجدت أم الدرداء فقالت تريد الحج العام قال قلت نعم فقالت فادع لنا بخير فإن النبي ﷺ كان يقول إن دعوة المسلم مستجابة لأخيه بظهر الغيب عند رأسه ملك موكل كلما دعا لأخيه بخير قال آمين ولك بمثل فخرجت إلى السوق فألقى أبا الدرداء فقال لي مثل ذلك يأثره عن النبي ﷺ
Its isnad is sahih. Safwan bin Abdullah bin Safwan is trustworthy, and his hadith is found in Muslim and the four Sunan compilers.The hadith was narrated by Muslim (4/2094), Bukhari in Adab al-Mufrad (215, no. 625), and Ibn Abu Shayba (7/22) in the chapter on supplication.