Musannaf Ibn Abi Shayba 3:4037

Abu Usama narrated to us, from Uthman ibn Ghiyath, who said: Abu Uthman narrated to me, saying: I saw a man come while Umar ibn al-Khattab was in the dawn prayer, so he would pray the two rak'ahs at the side of the mosque. Then he would enter into the prayer with the people.