What is the ruling on one who prays Maghrib behind an imam who is praying 'Isha'

Question:
We prayed Maghrib and 'Isha' combined early because of the rain, and a man entered while we were praying 'Isha' in the first rak'ah. Should he pray it as 'Isha' and then pray Maghrib afterwards? Or should he pray it as Maghrib? How is that done? We ask for guidance. May Allah reward you with good.

Answer:
Should he pray it as 'Isha'? No, he does not pray it as 'Isha', because 'Isha' is not prayed except after Maghrib; the order between the obligatory prayers is obligatory. If he entered with them intending Maghrib, there is no harm in that, if Allah wills. However, when the imam stands for the fourth rak'ah, he sits down and does not follow him, because the prayer of Maghrib is three rak'ahs. Then he has a choice: if he wishes, he says the taslim for himself, and if he wishes, he waits until the imam comes for the fourth rak'ah and says the taslim, and he says the taslim with him. Yes.