Maliks Muwatta Book 9, Hadith Number 69.
Section : Prayer on the Prophet, may Allah Bless Him and Grant Him Peace.
Yahya related to me from Malik from Abdullah ibn Abi Bakr ibn Hazim from his father that Amr ibn Sulaym az-Zuraqi said, “Abu Humayd as-Saidi told me that they asked the Messenger of Allah, may Allah bless him and grant him peace, how they were to ask for blessings upon him and he replied that they should say, ‘O Allah, bless Muhammad and his wives and his descendants as You blessed the family of Ibrahim, and give baraka to Muhammad and his wives and his descendants as You gave baraka to the family of Ibrahim. You are worthy of Praise and Glorious.'”
Allahumma salli ala Muhammad wa azwajihi wa alihi kama sallaita ala ali Ibrahim, wa barakaala Muhammad wa azwajihi wa alihi kama baraktaalaali Ibrahim, innaka Hamidu’m-Majid.