Entering the mosque (while I am a traveler) and the imam is praying the 'Isha' prayer, so I joined him with the intention of Maghrib and separated from him in the third rak'ah

The questioner : My last question is a question, meaning, regarding the prayer. I heard a fatwa from you, and perhaps I misunderstood it, because I discussed it with my brother Hashim. I told him, meaning, one day I entered the mosque of 'Umar ibn al-Khattab in Zarqa, and it was the 'Isha' prayer and I was a traveler. So 'Iyad, who works with my father, said to me: Pray 'Isha' with them, but intend Maghrib, and in the third rak'ah, without the tashahhud, when I reach the third rak'ah, I say the taslim, then I stand up and say the takbir and join the imam in the fourth rak'ah. Then, meaning, when the imam says the taslim, I complete with one rak'ah and say the taslim. And when I told my brother Hashim, he said to me...

The Shaykh : Excuse me, do you mean you complete with one rak'ah, meaning you would have prayed 'Isha' as two rak'ahs? Or four?

The questioner : I mean I prayed three rak'ahs with the imam...

The Shaykh : Three rak'ahs of Maghrib?

The questioner : Yes, with the imam.

The Shaykh : I am asking you: Did you pray 'Isha' as two rak'ahs or four?

The questioner : Two rak'ahs.

The Shaykh : That is it. So, what was the disagreement that occurred between you and Hashim?

The questioner : I told him, I heard the fatwa this way, or I asked you by phone, I remember... He said to me, perhaps it is not like that, and perhaps you are mistaken.

The Shaykh : It is so. When you prayed Maghrib for three rak'ahs and separated from the imam in the fourth rak'ah, you sat and said the taslim, you exited the Maghrib prayer.

The questioner : I separated from the imam in the third rak'ah...

The Shaykh : I am speaking. I prayed Maghrib for three rak'ahs and stood up and caught up with the imam's prayer for one rak'ah. You intended to pray 'Isha', so when the imam said the taslim, you stood up and performed a second rak'ah and said the taslim.

The questioner: Yes.

The Shaykh : This is a mistake, because when you followed the resident imam, your prayer changes to the prayer of a resident. There is no difference between you in this situation and if you had caught up with the 'Isha' prayer with the resident imam at its beginning. What do you do if you entered the mosque and the 'Isha' prayer was established, and you wanted to pray the 'Isha' prayer? The imam started the prayer from here, and you started with him. So when he stood up for the third rak'ah, do you stand up with him or not?

The questioner : I stand up with him.

The Shaykh : So, likewise here. Or before I tell you "likewise here," why do you stand up with him? Because you followed the resident imam, and there is no difference whether you followed the resident imam at the beginning of the prayer, or in the second rak'ah, or in the third rak'ah, or before the taslim. The minimum is that you catch up with the imam in the tashahhud before he says the taslim. As soon as you follow the resident imam and you are a traveler, your prayer changes from the prayer of a traveler to the prayer of a resident. Is that clear? Anything else.

The questioner : Peace be upon you.

The Shaykh : And upon you be peace. And this is a religious innovation, and perhaps it is not widespread among you? Because I hear it often from you. This means...