Musannaf Ibn Abi Shayba 3:4035

Abu Bakr narrated to us, he said: Hushaym narrated to us, he said: Husayn and Ibn ‘Awn informed us, from al-Sha‘bi, from Masruq: That he entered the mosque while the people were in the dawn prayer, and he had not prayed the two rak‘ahs. So he prayed them in a corner, then he entered with the people into their prayer.