What is the ruling on one who enters a mosque and has not prayed Maghrib, but finds them praying 'Isha'?

The questioner : This is a questioner asking a fiqh question. He says: If a man comes to the mosque and has not prayed Maghrib, and finds a congregation praying 'Isha, or shortening the prayer, what is the ruling in that case?

The Shaykh : I mean shortening the prayer. Yes, he has not prayed Maghrib, he entered. This is not a problem in reality, yes.
If he enters with people who are praying 'Isha, while he is praying Maghrib and they are shortening, there is absolutely no problem. Because if he joins them in the first rak'ah and the imam concludes the prayer, he stands and performs one rak'ah, so he has completed three. There is no issue with this; there is only a difference in intention between the imam and the one praying behind him, and the correct view is that it does not affect the validity. However, the problem is if these people are praying 'Isha as four rak'ahs, and he has not prayed Maghrib, so he joins them. If he joins them in the second rak'ah or any after that, there is also no issue. If he joins in the second, when does he conclude the prayer? When the imam concludes. If he prays three, and someone says this man who prayed three recited the tashahhud in the first rak'ah and missed the tashahhud in the second, we say there is no harm. Just as in the example, if he comes and the imam is praying Zuhr and he has missed a rak'ah, he will recite the tashahhud in the first rak'ah and place the tashahhud in the second rak'ah; this does not matter. But if he joins the imam in the first rak'ah, and the imam is praying four, and this man will pray Maghrib as three, what does he do? Does he follow the imam and pray four? I answer him: No. Good. Does he wait for the imam, meaning he sits while the imam is standing for the fourth, and this man waits and concludes the prayer with him? This is possible, but better than this is that he intends to pray individually when the imam stands for the fourth, and he recites the tashahhud and concludes the prayer to catch up with the imam in the fourth rak'ah. Shaykh al-Zamakhshari mentioned that there is no harm in something like this, and we also see that there is no harm in it, even though some scholars do not agree with it, but this is the stronger position, if Allah wills.