Musannaf Ibn Abi Shayba 3:2475

Abu Bakr narrated to us, he said: Hushaym narrated to us, he said: Mughira informed us, from Ibrahim, regarding a man who comes to the mosque and sees that they have already prayed, so he assumes the prayer and prays two rak'ahs of the obligatory prayer. Then the iqamah for the prayer is called. He said: He enters with the Imam in his prayer. When he has prayed two rak'ahs with the Imam, he then performs the taslim, and he makes the remaining two rak'ahs with the Imam voluntary.