The ruling on praying Maghrib behind one who is praying 'Isha'

Question:
A man entered the mosque to perform the Maghrib prayer during the time of Maghrib. The time was indeed the time of Maghrib, and the imam had combined the Maghrib and 'Isha' prayers because the night was rainy. When the man entered, he found that the imam had already begun the 'Isha' prayer, and the man was unaware of this. So he entered with him intending the Maghrib prayer until the imam completed the prayer, and the man thought that the imam had made a mistake, because he assumed it was the Maghrib prayer. What is the ruling on the man's prayer?

Answer:
As for if he knew, he should intend it as Maghrib, sit in the third rak'ah, and wait for the imam until he says the taslim, then say the taslim with him, and then pray 'Isha' afterwards.
As long as he intended it as Maghrib and stood up with the imam out of ignorance or forgetfulness, his prayer is valid; because this extra rak'ah occurred due to ignorance. Otherwise, it is obligatory for him to sit and not stand up. If the ma'mum stand up with the imam in the extra rak'ah due to their ignorance or forgetfulness, the prayer is valid. For example, if he prayed 'Isha' five rak'ahs, or prayed Maghrib four rak'ahs, or prayed Fajr three rak'ahs, and some of the ma'mum stood up with him out of his ignorance or forgetfulness, their prayer is valid. But it is obligatory for them not to stand up with him in the excess; they should sit. When he says the taslim, they say the taslim with him, while drawing attention to the mistake. This is what is obligatory for him...