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

Question : The questioner says: A group of people are praying Maghrib and 'Isha together as a combined prayer. A man joined them, catching up with the congregation while believing they were praying Maghrib, but they were actually praying 'Isha. He completed the prayer with them. What is the ruling in this case? And how is his prayer?

Answer : This is the case of one who catches up with a group combining Maghrib and 'Isha. He joined them in the 'Isha prayer, thinking it was Maghrib, and he intended it to be Maghrib. He is in one of two situations: either he joined them in the first rak'ah, or he joined them after that. If he joined them in the first rak'ah, and they were completing the prayer (i.e., praying four rak'ahs), then it is obligatory for him, when the imam stands for the fourth rak'ah, to intend to separate from him. He should sit, recite the tashahhud, and say the taslim. Then he should stand up and follow the imam in the remainder of the 'Isha prayer.
As for if he catches up with the imam in the second, third, or fourth rak'ah, he should complete it as Maghrib. If he catches up with the imam in the second rak'ah, he says the taslim with him. If he catches up with him in the third rak'ah, he prays one rak'ah after him. If he catches up with him in the fourth rak'ah, he prays two rak'ahs after him. It will be Maghrib for him, and 'Isha for the imam and those with him.