Completing the prayer for the traveler who joins the resident imam

Question:
This questioner says: I do not cover myself during prayer. I was traveling when the time for the 'Asr prayer came, and as I entered the mosque, I found the imam in the first tashahhud. I joined him with the intention of shortening the 'Asr prayer to two rak'ahs. When he finished the prayer with the taslim, I did the same. Afterwards, someone told me that this is not permissible and that I must complete the prayer. I ask for your guidance.

Answer:
Yes. That is the case. Whoever joins the imam who is completing the prayer must complete it, even if he catches only the end of it, or even if he catches only the end of the prayer. Whoever prays with the imam who is completing the prayer is required to complete it.
It is not permissible for him to shorten it, because following the imam is obligatory. The Prophet (peace and blessings be upon him) said: So do not differ with him. It is not permissible for the follower who shortens the prayer to shorten it if he prays with the resident imam; it is not permissible for him to shorten behind him. Rather, he must follow him, following the imam. Yes.