Sahih Muslim Book 4, Hadith Number 807.

Abu Humaid as-Sa’idi reported: They (the Companions of the Holy Prophet) said: Apostle of Allah, how should we bless you? He (the Holy Prophet) observed: Say: “O Allah! bless Muhammad, his wives and his offspring as Thou didst bless Ibrahim, and grant favours to Muhammad, and his wives and his offspring as Thou didst grant favours to the family of Ibrahim; Thou art Praiseworthy and Glorious.”