Musnad Ahmad 1061:3

I read to Abd al-Rahman: Malik, from Abdullah ibn Abi Bakr, from his father, from Amr ibn Sulaym, that he said: Abu Humayd al-Sa'idi informed me that they said: "O Messenger of Allah, how should we send blessings upon you?" The Messenger of Allah, peace and blessings of Allah be upon him, said: "O Allah, send blessings upon Muhammad, his wives, and his offspring, as You sent blessings upon the family of Ibrahim, and bless Muhammad, his wives, and his offspring, as You blessed the family of Ibrahim. Indeed, You are Praiseworthy, Glorious."