Hadith 1303:2
Yazid ibn Harun narrated to us, Abd al-Malik informed us, from Abu al-Zubayr, from Safwan ibn Abdullah ibn Safwan—and al-Darda’ was his wife—that he came to them and found Umm al-Darda’. She said to him, "Do you intend to perform Hajj this year?" He said, "Yes." She said, "Then supplicate for us with good, for the Prophet, peace and blessings of Allah be upon him, used to say: 'Indeed, the supplication of a Muslim person for his brother in his absence is answered. At his head is an angel appointed for him; every time he supplicates for his brother with good, the angel says: "Amin, and for you the same."'" He said: So 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.
حدثنا يزيد بن هارون، أخبرنا عبد الملك، عن أبي الزبير، عن صفوان بن عبد الله بن صفوان - وكانت تحته الدرداء - فأتاهم فوجد أم الدرداء فقالت له: أتريد الحج العام؟ فقال: نعم قالت: فادع لنا بخير، فإن النبي صلى الله عليه وسلم كان يقول: " إن دعوة المرء المسلم مستجابة لأخيه بظهر الغيب، عند رأسه ملك موكل به كلما دعا لأخيه بخير قال: آمين ولك بمثل " قال: فخرجت إلى السوق فلقيت أبا الدرداء فحدثني عن النبي صلى الله عليه وسلم بمثل ذلك