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 asked the Prophet (peace and blessings be upon him) about sending blessings upon him. We said: "The 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 and the family of Ibrahim; indeed, You are the Praised, the Glorious. And send down Your blessings upon Muhammad and the family of Muhammad as You sent down Your blessings upon the family of Ibrahim; indeed, You are the Praised, the Glorious." Ka'b (may Allah be pleased with him) said: And we would also say: "And upon us along with them (send down Your mercy)."
Hadith Referenceالمعجم الكبير للطبراني / باب الكاف / 16337