Musannaf Ibn Abi Shayba 3:6257

Muhammad ibn Bishr narrated to us, from Mujammi' ibn Yahya, from Uthman ibn Mawhab, from Musa ibn Talhah, from his father, who said: We said, "O Messenger of Allah, we have already learned the greeting (salam) upon you, so how is the prayer (salat)?" He said, "Say: O Allah, send prayers upon Muhammad and upon the family of Muhammad, as You sent prayers upon Ibrahim and the family of Ibrahim. Indeed, You are Praiseworthy, Glorious."