Mishkat al-Masabih #864

Abu Humaid as-Sa‘idi said that when some people asked God’s Messenger how they should invoke blessings on him he told them to say, “O God, bless Muhammad, his wives and his offspring as Thou didst bless Abraham; and grant favours to Muhammad, his wives and his offspring as Thou didst grant favours to the family of Abraham. Thou art indeed praiseworthy and glorious.” (Bukhari and Muslim.)