Abd al-Rahman bin Abi Layla (may Allah have mercy on him) said: Sayyiduna Ka'b bin Ujrah (may Allah be pleased with him) met me and said: "Shall I not give you a gift?" I said: "Certainly, grant it to me." He said: We said: "O Messenger of Allah, how should blessings be sent upon you, the People of the House, for Allah, the Mighty and Majestic, has taught us how to send salutations?" 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. O Allah, 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."
Hadith Referenceالمعجم الكبير للطبراني / باب الكاف / 16333