Sayyiduna Ka'b bin Ujrah (may Allah be pleased with him) said: When this verse was revealed: "Indeed, Allah and His angels send blessings upon the Prophet" [Surah al-Ahzab: 56], I said: "O Messenger of Allah, the salutation we have come to know; how shall we send blessings upon you, O Messenger of Allah?" He (peace and blessings be upon him) said: "Say: O Allah, send down Your mercy upon Muhammad and the family of Muhammad as You sent down Your mercy upon Ibrahim and the family of Ibrahim; indeed, You are the Praised, the Glorious."
Hadith Referenceالمعجم الكبير للطبراني / باب الكاف / 16324