Is it permissible for a traveler to shorten the prayer (perform qasr) if he catches the last two rak'ahs of a four-rak'ah prayer and intends to shorten it?

If a traveler prays behind a resident, he must complete the prayer with four rak'ahs. This is because following the Imam is obligatory, and shortening the prayer during travel is a Sunnah, not an obligation. The Companions acted upon this by completing their prayers behind Uthman in Mina, as narrated by Imam Ahmad from Ibn Abbas that this was the Sunnah of Abu Al-Qasim (peace be upon him).