Sayyidah Umm al-Darda' (may Allah be pleased with her) narrates: My master (husband) narrated to me that he heard the Messenger of Allah (peace and blessings be upon him) saying: "Whoever supplicates for his brother in his absence, the angel appointed (over him) says 'Amin' to it and says: 'O supplicant, and for you the same.'"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الاستسقاء / 6431