Musannaf Ibn Abi Shaybah 4934 — Those who said: he completes what

Ibrahim said: If a man comes to the mosque and thinks that the people have already prayed, so he begins the obligatory prayer, and he has completed only two rak'ahs when the iqamah is called for prayer, he should now join the imam in his prayer. When the imam has prayed two rak'ahs (his four are complete), he should say the salam and then continue with the imam in the rest of the prayer, making the last two rak'ahs with the imam voluntary.