Musnad Ahmad 1016:3

Yahya ibn Sa'id narrated to us, he said: And Shu'bah narrated to us, Sa'd ibn Ibrahim narrated to me, Hafs ibn 'Asim narrated to me, from Malik ibn Buhaynah, that the Prophet, peace and blessings of Allah be upon him, saw a man praying the two rak'ahs of Fajr while the prayer had already been established. When he finished the prayer, the people gathered around him. The Prophet, peace and blessings of Allah be upon him, said: "Is the morning [prayer] four [rak'ahs]?"