What is the ruling on someone who caught the Dhuhr prayer in congregation while traveling after the Tashahhud, and prayed two rak'ahs?

A traveler praying behind a resident must complete the prayer, whether he caught it from the beginning, or caught two rak'ahs, or one rak'ah. If the traveler, who is led by a resident, says the taslim after only two rak'ahs in a four-rak'ah prayer, his prayer becomes void and he must repeat it.