The ruling on a traveler who prays two rak'ahs behind a resident and does not know until later

If a person is traveling and finds people praying the midday prayer while they are residents, and he did not know that they were residents, and he only caught up with two rak'ahs before he said the closing salam, and after the prayer he learned that they had prayed four rak'ahs, what is the ruling?
It is obligatory for him to repeat this prayer from which he said the closing salam, because when a traveler joins the imam, his ruling becomes the ruling of the imam.