Musnad Abu Ya'la al-Mawsili 16:2

Mansur ibn Abi Muzahim narrated to us, Ibrahim ibn Sa'd narrated to us, from his father, from Hafs ibn 'Asim, from Ibn Buhaynah, who said: The prayer was established, and the Prophet, peace and blessings of Allah be upon him, passed by a man praying two rak'ahs before the morning prayer. He spoke to him about something, and we did not know what it was. When we turned away, we surrounded him, asking him what the Prophet, peace and blessings of Allah be upon him, had said. He said: "He said to me: 'It is imminent that one of you will pray the morning prayer as four.'"