The ruling on one who intends to pray Maghrib behind an imam who is praying 'Isha'

Question:
He says: Honorable Shaykh, may Allah grant you success. A person joined the congregation while they were praying 'Isha', and his intention was to pray Maghrib. When the imam stood for the fourth rak'ah, he said the taslim for himself, then caught up with them with the intention of praying 'Isha' in the last rak'ah. So what is the ruling on his action?

Answer:
Or is his Maghrib prayer valid? As long as he sat and said the taslim for himself, he has a choice. If he wishes, he says the taslim for himself, meaning he performs the final tashahhud, and then if he wishes, he says the taslim for himself, or if he wishes, he waits until the imam says the taslim. But his Maghrib prayer is valid. As for 'Isha', no, the prayer is not divided between two prayers, some of it being Maghrib and some of it being 'Isha'. That is not valid. Yes. He must repeat the 'Isha' prayer. Yes.