Sayyidah Umm Unays bint al-Hasan bin Ali (may Allah be pleased with them both) narrates from her father, who said: The Companions said: "O Messenger of Allah, tell us about the saying of Allah, the Mighty and Majestic: 'Indeed, Allah and His angels send blessings upon the Prophet' [Surah al-Ahzab: 56]." He (peace and blessings be upon him) said: "This is something hidden; had you not asked me about it, I would not have told you. Allah, the Mighty and Majestic, has appointed two angels to me. I am not mentioned before any Muslim servant who then sends blessings upon me except that those two angels say: 'May Allah forgive you,' and Allah and His angels say in reply to those two angels: 'Amin.' And no one sends blessings upon me except that those two angels say: 'May Allah forgive you,' and Allah and His angels say in reply to those two angels: 'Amin.'"
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 2740