Musnad Ahmad 1016:10

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, and Hajjaj narrated to us, Shu‘bah informed us, from Sa‘d ibn Ibrahim, from Hafs ibn ‘Asim and others—Hajjaj said in his narration: I heard Hafs ibn ‘Asim ibn ‘Umar ibn al-Khattab, from Malik ibn Buhaynah, that a man entered the mosque while the prayer had been established, so he prayed the two rak‘ahs of Fajr. When the Messenger of Allah, peace and blessings of Allah be upon him, finished, the people gathered around him. He said, "Is the morning [prayer] four [rak‘ahs]?"