Musnad Ahmad 1017:54

Yazid ibn Harun narrated to us, Ismail informed us, from Abu Dawud al-A'ma, from Buraydah al-Khuza'i, who said: We said, "O Messenger of Allah, we have learned how to greet you, so how should we send blessings upon you?" He said: "Say: 'O Allah, place Your blessings, Your mercy, and Your benedictions upon Muhammad and upon the family of Muhammad, just as You placed them upon the family of Ibrahim. Indeed, You are Praiseworthy, Glorious.'"