Sahih Bukhari Volume 4, Book 55, Hadith Number 589.
Narated By Abdur-Rahman bin Abi Laila : Ka’b bin Ujrah met me and said, “Shall I not give you a present I got from the Prophet?” ‘Abdur-Rahman said, “Yes, give it to me.” I said, “We asked Allah’s Apostle saying, ‘O Allah’s Apostle! How should one (ask Allah to) send blessings on you, the members of the family, for Allah has taught us how to salute you (in the prayer)?’ He said, ‘Say: O Allah! Send Your Mercy on Muhammad and on the family of Muhammad, as You sent Your Mercy on Abraham and on the family of Abraham, for You are the Most Praise-worthy, the Most Glorious. O Allah! Send Your Blessings on Muhammad and the family of Muhammad, as You sent your Blessings on Abraham and on the family of Abraham, for You are the Most Praise-worthy, the Most Glorious.'”