If one prays Maghrib with an imam who is praying 'Isha', does he leave him in the third rak'ah or does he recite the tashahhud and wait for him?

The questioner:
May Allah reward you. This is a question from a listener, M-M from al-Dawasi, who says: I caught up with the imam while he was praying 'Isha, so I joined him with the intention of Maghrib, because I had not prayed Maghrib before. When he stood up to perform the fourth rak'ah, I intended to separate from him, recited the final tashahhud, and then said the taslim.

The Shaykh:
Yes, the question.

The questioner:
He says: I caught up with the imam while he was praying 'Isha, so I joined him with the intention of Maghrib, because I had not prayed Maghrib before. When the imam stood up to perform the fourth rak'ah, I intended to separate from him, recited the final tashahhud, and then said the taslim. What is the ruling in this situation? Is it better to do what I did, or should I wait until the imam completes the fourth rak'ah, then recite the tashahhud and say the taslim with him?

The Shaykh:
What you did is permissible. If you sat down, left the fourth rak'ah, recited the tashahhud, and said the taslim for yourself, that is fine. And if you recited the final tashahhud and waited until the imam said the taslim, then you said the taslim with him, that is also permissible. However, despite all this, the best and most cautious course is to pray Maghrib before 'Isha. You should pray Maghrib alone or with a group of people like you who have not prayed Maghrib, and then join the congregation for the remainder of 'Isha. This is the best and most cautious approach. Yes.