What is the ruling on praying Maghrib with an imam who is praying 'Isha'?

Question:
A traveling man passed through Madinah at the time of 'Isha, so he joined the imam in the 'Isha prayer, but he had not yet prayed Maghrib. What is the ruling?

Answer:
If he joined him in 'Isha with the intention of 'Isha, he should complete it as a voluntary prayer; because 'Isha is not valid before Maghrib according to the correct view, due to the obligation of sequence. So he completes it as a voluntary prayer, then prays Maghrib, then prays 'Isha.
As for if he entered it with the intention of Maghrib, and sat in the third rak'ah until the imam said the closing salam and he said the closing salam with him with the intention of Maghrib, then he prays 'Isha, the correct view is that it is valid, if Allah wills; because he performed it completely with the intention of Maghrib, and he is excused when he sat in the third rak'ah, as in the prayer of fear; the second row stands, the second row lags behind in some types of the prayer of fear until the first row rises from its prostration, then the second row prostrates; because he is standing guarding until the first row rises from its prostration, and as the group waits in another type, he prays with one group two rak'ahs, and with another group two rak'ahs, but he prays with the first group one rak'ah, then they complete it for themselves, then they leave, then he waits, then the second group comes and he prays with them the remaining rak'ah, then they say the closing salam and complete it for themselves, and there are several types of the prayer of fear.