Sahih Bukhari Volume 4, Book 55, Hadith Number 588.

Narated By Abu Humaid As-Sa’idi : The people asked, “O Allah’s Apostle! How shall we (ask Allah to) send blessings on you?” Allah’s Apostle replied, “Say: O Allah! Send Your Mercy on Muhammad and on his wives and on his off spring, as You sent Your Mercy on Abraham’s family; and send Your Blessings on Muhammad and on his offspring, as You sent Your Blessings on Abraham’s family, for You are the Most Praise-worthy, the Most Glorious.”