Musannaf Ibn Abi Shayba 35:327

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