Musnad Ahmad 457:10

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, who said: And Muhammad ibn Ibrahim ibn al-Harith al-Taymi narrated to me—regarding the prayer upon the Messenger of Allah, may Allah bless him and grant him peace, when a Muslim person prays upon him in his prayer—from Muhammad ibn 'Abd Allah ibn Zayd ibn 'Abd Rabbihi al-Ansari, brother of Banu al-Harith ibn al-Khazraj, from Abu Mas'ud 'Uqbah ibn 'Amr, who said: A man came until he sat before the Messenger of Allah, may Allah bless him and grant him peace, while we were with him, and he said: "O Messenger of Allah, as for the greeting (salam) upon you, we have already known it, so how should we pray upon you when we pray in our prayer, may Allah bless you?" He said: The Messenger of Allah, may Allah bless him and grant him peace, remained silent until we wished that the man had not asked him. Then he said: "When you pray upon me, say: 'O Allah, send prayers upon Muhammad, the unlettered Prophet, and upon the family of Muhammad, as You sent prayers upon Ibrahim and the family of Ibrahim, and bless Muhammad, the unlettered Prophet, as You blessed Ibrahim and the family of Ibrahim; indeed, You are Praiseworthy, Glorious.'"