Musannaf Ibn Abi Shayba 3:6256

Khalid ibn Makhlad narrated to us, from Abdullah ibn Ja'far, from Yazid ibn al-Had, from Abdullah ibn Khabbab, from Abu Sa'id al-Khudri, who said: We said, "O Messenger of Allah, this greeting (salam) upon you, we have already learned it, so how is the prayer (salat) upon you?" He said, "Say: O Allah, send prayers upon Muhammad, Your servant and Your Messenger, as You sent prayers upon Ibrahim, and bless Muhammad as You blessed Ibrahim."