How does a traveler (musafir) perform prayer behind a resident imam?

If a traveler (musafir) follows a resident imam in prayer, he performs the prayer in full (Mevsılî, el-İhtiyâr, 1/80). This is because the Messenger of Allah (peace be upon him) said, “The imam is appointed to be followed.” (Bukhari, Salat, 18 [378]; Muslim, Salat, 77 [411]), indicating that the congregation’s prayer should be the same as the imam’s. If the traveler joins a resident imam during the prayer and gives salam (ends the prayer) before the imam completes it, the prayer he performed is not valid. In this case, when the person whose prayer is invalid repeats the same prayer alone, he performs it as two rak‘ahs, not four.

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu