Musannaf Ibn Abi Shayba 3:6254

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Mis’ar, from al-Hakam, from Ibn Abi Layla, from Ka’b ibn ‘Ujra, who said: We said, "O Messenger of Allah, we have already learned how to greet you (saying *as-salamu ‘alayk*), so how should we send blessings upon you?" He said: "Say: O Allah, send blessings upon Muhammad and upon the family of Muhammad, as You sent blessings upon Ibrahim and the family of Ibrahim; indeed, You are Praiseworthy, Glorious. And bless Muhammad and the family of Muhammad, as You blessed Ibrahim and the family of Ibrahim; indeed, You are Praiseworthy, Glorious."