Musnad Abu Ya'la al-Mawsili 42:391

On the authority of Abu Sa'id al-Khudri, he said: We said, "O Messenger of Allah, this is the greeting (salam) upon you, so how do we send blessings (salat) upon you?" He said: "Say: O Allah, send blessings upon Muhammad, Your servant and Your Messenger, just as You sent blessings upon Ibrahim, and bless Muhammad and the family of Muhammad, just as You blessed Ibrahim."