How a traveler prays with a congregation when he does not know that they are travelers

Question:
A letter from al-Hasa, sent by the listener Salem Sa'id Muhammad, containing a number of questions. In one of them, he says: A man came and found a group praying in the mosque. He is a traveler, and they are in the last two rak'ahs. This mosque is on the road between Makkah and Madinah, but he does not know: is this prayer shortened or complete? Does he complete the prayer? Or does he say the taslim with them? Does he intend to pray it shortened or complete? And what is best for him in this situation? May Allah reward you with good.

Answer:
It is obligatory for him to pray four rak'ahs, because the people of the mosque are praying four. So it is obligatory for him to pray four. If he catches two, he stands up and performs two after the taslim, because it is established from the Prophet (peace and blessings be upon him) that if a traveler prays with residents, he prays four, and he does not intend the two, but rather he prays four.
As for if he knows that they are travelers and prays two with them, there is no harm, or if they had signs of travel and he assumed they were travelers, and his assumption was correct, then his prayer is valid. But if he did not know that, then the default for the people of mosques is that they are residents, so he prays four with them, not two. If he catches two and they say the taslim, he stands up and performs the last two. Yes.

The presenter: May Allah reward you with good.