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. O you who believe, send blessings upon him and greet him with a full salutation" [Surah al-Ahzab: 56], we said: "O Messenger of Allah, this salutation we have come to know; how shall we send blessings upon you?" 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; indeed, You are the Praised, the Glorious; and send down Your mercy upon us along with them. And send down Your blessings upon Muhammad and the family of Muhammad as You sent down Your blessings upon Ibrahim and the family of Ibrahim; indeed, You are the Praised, the Glorious; and send down Your blessings upon us along with them. And peace be upon you, O Prophet, and the mercy of Allah and His blessings."
Hadith Referenceالمعجم الكبير للطبراني / باب الكاف / 16321