Ruling on the traveler praying behind the resident

The questioner : May Allah reward you, Shaykh! I entered a mosque while I was traveling. This mosque is in Madinah and is not on my route of travel. I entered while they were in the fourth and final rak'ah, so I prayed one rak'ah with them. Then I stood up for the second rak'ah, and it was the 'Asr prayer, so I shortened the prayer, even though they had completed it before me. What is the ruling?
The Shaykh : The default rule in Madinah is that those who pray there pray four rak'ahs,Meaning: completing the prayer. Therefore, he has erred. He must now repeat the prayer as four rak'ahs, and his first prayer is considered a voluntary prayer.