Musannaf Ibn Abi Shayba 3:4067

Ghundar narrated to us, from Shu'bah, from Yahya ibn Sa'id, who said: I heard al-Qasim say: If I did not pray them [the two rak'ahs of Sunnah of Fajr] until I prayed the Fajr [obligatory prayer], I would pray them after the rising of the sun.