What does a traveler do if he catches a unit of prayer with a congregation?

Question:
He says: I was traveling, and I was shortening the prayer, so I caught a congregation praying the 'Asr prayer. I prayed one unit with them, and the imam concluded the prayer, but I do not know if he was shortening or if he prayed the full prayer. How do you advise us if this situation recurs? May Allah reward you with good.

Answer:
If the people you prayed with were travelers, you complete one unit; because it appears that they are travelers and prayed two units. However, if you prayed with residents, you pray four units. When they conclude the prayer, you stand up and complete three units if you caught one, and if you caught two, you stand up and complete the remaining two; because it appears that the residents are praying four units.
The prescribed and obligatory action for a traveler who prays with residents is to pray four units. As for if you prayed with travelers, if you caught one unit, you stand up and complete the second unit; because it appears from their state that they prayed two units, that they are travelers. Yes.

The presenter: May Allah reward you with good.