Musnad Ahmad 457:5

'Uthman ibn 'Umar narrated to us, Malik informed us, from Nu'aym al-Mujmir, from Muhammad—meaning ibn 'Abd Allah—from Abu Mas'ud, who said: It was said, "O Messenger of Allah, how do we send blessings upon you?" He said: "Say: 'O Allah, send blessings upon Muhammad and upon the family of Muhammad, and bless Muhammad and the family of Muhammad, as You blessed Ibrahim among the worlds. Indeed, You are Praiseworthy, Glorious.'" I read this hadith to 'Abd al-Rahman: Malik, from Nu'aym ibn 'Abd Allah, that Muhammad ibn 'Abd Allah ibn Zayd informed him, from Abu Mas'ud.