What is the ruling on shortening the prayer for a traveler who prayed behind an imam who completed the prayer, while he thought the imam was shortening the prayer?

The questioner : May Allah reward you, O Shaykh: One of the brothers is asking, saying: I entered a mosque while I was traveling, and in fact, I entered when they were in the fourth and final rak'ah, so I prayed with them one rak'ah. It was the 'Asr prayer, then I stood up and I owed the second rak'ah, meaning I shortened the prayer even though they had completed it before me. What is the ruling, O Shaykh?

The Shaykh : As for the one who prayed behind people who were praying four rak'ahs, while he thought they were shortening the prayer, did it become clear to him that they prayed four rak'ahs?

The questioner : Before or after, I mean?

The Shaykh : After, before it became clear to him.

The questioner : After it became clear to him.

The Shaykh : That they prayed four rak'ahs.

The questioner : Yes, it became clear.

The Shaykh : Now, is this a mosque where it is customary for travelers to pray in it, like the mosques on the routes at the stations?

The questioner : No, he entered the city.

The Shaykh : The city, so the original ruling in the city is that those who pray in it pray four rak'ahs.

The questioner : Four rak'ahs, yes.

The Shaykh : So he was negligent, and he must now repeat the prayer with four rak'ahs, and his first prayer becomes a voluntary prayer that brings him closer to Allah (Mighty and Majestic).

The questioner : May Allah reward you.

The Shaykh : This was conveyed to him.

The questioner : Your Eminence, what is the ruling on the device.

The Shaykh : Here, one person asked on the right and one on the left.