Musnad Ahmad 1016:8

Ya'qub narrated to us, my father narrated to us, from his father, Hafs ibn 'Asim narrated to us, from 'Abdullah ibn Malik ibn Buhaynah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) passed by a man while the prayer had been established, and he was praying the two rak'ahs before the dawn prayer. He said something to him, and we did not know what it was. When we turned away, we surrounded him, saying: "What did the Messenger of Allah say to you?" He said: "He said to me: 'It is imminent that one of you will pray the morning prayer as four [rak'ahs].'"