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

The questioner : Honorable Shaykh, if a person misses the Maghrib prayer because he was sleeping or excused, and he wakes up to find the people praying the 'Isha' prayer, and he enters the mosque while they are praying, what should he do? We ask for a detailed explanation of this issue, as there has been much discussion about it. May Allah reward you with good.

The Shaykh : Yes, if a person enters the mosque while the people are praying the 'Isha' prayer and he has not prayed the Maghrib prayer, the scholars (may Allah have mercy on them) differed on this. Some said he should pray Maghrib alone and then join the imam in the remainder of the 'Isha' prayer. Some said he should pray 'Isha' first, following the congregation, and then pray Maghrib. Some said he should join them with the intention of Maghrib. If he joins at the beginning of the prayer, then when the imam stands for the fourth rak'ah, he sits, recites the tashahhud, and says the taslim, and then joins the imam in the remainder. If he joins the imam in the second rak'ah,

The questioner : what does he do? He says the taslim with him. If he joins the imam in the third rak'ah, he makes up one rak'ah. If he joins him in the fourth,

The Shaykh : he makes up two rak'ahs. This last opinion is the stronger one (rajih), which the Shaykh al-Islam Ibn Taymiyyah chose. That means you join them with the intention of Maghrib, and then when you have completed the three rak'ahs and the imam stands for the fourth, you sit, recite the tashahhud, and join him in the remainder of the 'Isha' prayer. Yes, this is the stronger opinion. Yes, the reason for preferring this opinion is that you obtain the reward of the congregation. Unlike if you prayed alone and performed the two prayers in their proper order, or if you started with 'Isha'. However, there is a concern in this view, which is actually not a concern, and that is that you separate from your imam when he stands for the fourth rak'ah. Separating from the imam for an excuse is permissible. For this reason, the man who joined Mu'adh ibn Jabal (may Allah be pleased with him) in the 'Isha' prayer, and Mu'adh recited Surat al-Baqarah, separated from him and intended to pray alone. This person's praying alone was due to an excuse, because he could not pray Maghrib as four rak'ahs. So this opinion is the stronger one. Honorable Shaykh, how? He must say the taslim when he separates from the imam in the fourth rak'ah. And if he recites the tashahhud, let him say the taslim. Yes.