The Book of Prayers (Kitab Al-Salat)
Muslim :: Book 4 : Hadith 804
Ibn Abi Laila reported: Ka'b b. 'Ujra met me and said: Should I not offer you a present (and added): The Messenger of Allah (may peace be upon him) came to us and we said: We have learnt how to invoke peace upon you; (kindly tell us) how we should bless you. He (the Holy Prophet) said: Say:" O Allah: bless Muhammad and his family as Thou didst bless the family of Ibrahim. Verily Thou art Praiseworthy and Glorious, O Allah."