Sahih Muslim Book 4, Hadith Number 804.

Chapter : The Tashahhud in prayer.

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.”