What does the ma'mum do if he intends Maghrib while the imam is praying 'Isha'

Q: It may happen in the combining of Maghrib and 'Isha' (for rain) that some of the congregation arrive while the imam is praying 'Isha', so they enter with the imam thinking he is praying Maghrib. What is required of them?

A: They must sit after the third rak'ah, recite the tashahhud and supplication, then say the taslim with him. Then they pray 'Isha' afterwards, in order to obtain the virtue of the congregation and fulfill the obligatory order. If he had preceded them by one rak'ah, they pray the remainder with him intending Maghrib, and it suffices for them as Maghrib.
If he had preceded them by more than that, they pray with him what they catch up with, then they make up what remains for them. Likewise, if they know that he is in 'Isha', they enter with him intending Maghrib and do what we mentioned, then they pray 'Isha' afterwards, according to the more correct of the two opinions of the scholars.