The Ruling on Praying Maghrib with the Imam of 'Isha'

Question:
A person from Makkah says: Honorable Shaykh, the time for the Maghrib prayer passed, and the time for the 'Isha' prayer entered. I prayed with the 'Isha' imam with the intention of Maghrib. Is it permissible for me to sit until the imam reaches the fourth rak'ah, so that I pray and say the taslim with him? Or should I sit for the final tashahhud, then say the taslim and catch up with the imam in the fourth rak'ah, which would count as the first rak'ah of the 'Isha' prayer? And does this count as opposing the mosque imam, given that the ma'mum follows the imam?

Answer:
One prayer is not divided between two prayers. It is not divided between Maghrib and 'Isha' as you mentioned. Praying Maghrib first, either alone or with a group like you who are praying Maghrib, and then joining the imam in the remainder of the 'Isha' prayer, is the more preferable and better course. As for what you did, entering with the imam who is praying 'Isha' with the intention of Maghrib, this is permissible. When he stands for the fourth rak'ah, you sit for the final tashahhud and perform the final tashahhud. Then you have a choice: either say the taslim for yourself, or wait until the imam says the taslim and you say it with him. Then you pray 'Isha' afterwards.