Musannaf Ibn Abi Shayba 3:4041

Ubayd Allah ibn Musa narrated to us, from Uthman ibn al-Aswad, from Mujahid, who said: If you enter the mosque while the people are in the morning prayer (Fajr) and you have not prayed the two rak'ahs of Fajr, then pray them, even if you think that the first rak'ah will pass you by.