Musnad Ahmad 31:443

'Abd al-Malik ibn 'Amr narrated to us, 'Abdullah ibn Ja'far al-Zuhri narrated to us, from Yazid ibn 'Abdullah 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 do we send blessings (salah) upon you?" He said: "Say: 'O Allah, send blessings upon Muhammad, Your servant and Your Messenger, as You sent blessings upon Ibrahim, and bless Muhammad and the family of Muhammad, as You blessed Ibrahim and the family of Ibrahim.'"