How to Pray When a Traveler Prays Behind a Resident, and Vice Versa

Question:
The first question: If I am traveling from Riyadh to Qassim and the midday prayer becomes obligatory for me while I am inside the city, do I shorten the midday prayer and combine it with the afternoon prayer, or do I pray the midday prayer as four units and shorten the afternoon prayer?

Answer:
A traveler shortens the prayer as long as he is traveling. However, if he prays behind an imam who completes the prayer, it is obligatory for him to complete it. The Prophet (peace and blessings be upon him) said, “The imam is appointed to be followed.” The Salaf, when they were traveling and prayed behind someone who completed the prayer, would complete it and not shorten it behind the imam.
Likewise, if the imam is a traveler and prays with residents, he shortens the prayer. But when he concludes the prayer with the taslim, they stand up and complete the prayer for themselves. This is because the Prophet (peace and blessings be upon him), during the Battle of the Conquest, when he prayed in the Sacred Precinct and the people prayed behind him, said, “O people of Makkah, complete the prayer for yourselves, for we are a traveling people.” Yes.