The ruling on one who prays Maghrib with a group who are praying 'Isha'

Question:
If a person catches up with a congregation in the 'Isha' prayer while they are shortening it, and he joins them in the second rak'ah with the intention of praying Maghrib, and the imam concludes the prayer, and the one who was behind stands up and performs the third rak'ah; is his prayer valid with the intention of Maghrib?

Answer:
This is a question for someone who missed the Maghrib prayer and found a group praying 'Isha', and he caught up with them in the third rak'ah, so the imam prayed the remaining two rak'ahs, then concluded the prayer, then this person stood up and performed the third rak'ah. Does that suffice for him?
The correct view is that it suffices for him if he intended Maghrib. He joins them with the intention of Maghrib, then prays two rak'ahs with the imam, the last two rak'ahs, then stands up and prays the third rak'ah, and that suffices for him. The difference in intention does not affect the validity; rather, it is the difference in actions that affects it.
The Prophet (peace and blessings be upon him) said: to the end of it. He mentioned the actions and the words, but did not address the intentions. So the place of the intention is the heart. If a person comes and the people are praying 'Asr, and he has not prayed Zuhr, he prays 'Asr with them with the intention of Zuhr. When the imam concludes the prayer, he performs 'Asr after that so that he does not miss the congregation, and so that he does not put 'Asr before Zuhr. Rather, he puts Zuhr first, as Allah has arranged it, and intends it with those who are praying 'Asr. When the imam concludes the prayer after that, he performs 'Asr, either alone, or with those who are present who are obligated to pray 'Asr.
Likewise, here, if a person comes and the people are praying 'Isha, and they have entered the third rak'ah, and he intends Maghrib; then when the imam concludes the prayer, he stands up until he performs the third rak'ah. And so, if they are shortening the 'Isha prayer and he comes to them, he prays 'Isha with them with the intention of Maghrib. When the imam concludes the prayer, the one who owes Maghrib stands up and prays the third rak'ah with the intention of Maghrib, and that suffices for him. He has his intention, and they have their intention.
There remains another scenario, which is: if a person joins them from the beginning of the prayer, and he owes the Maghrib prayer, and they are praying 'Isha in a non-traveling state. Does he pray with them with the intention of Maghrib, even if the actions differ, or does he not pray with them? There are two opinions among the scholars:
Some said: He prays with them a voluntary prayer, then prays Maghrib after that, and 'Isha. And some of the scholars said: There is no harm in praying with them, then sitting in the third rak'ah with the intention of Maghrib. When the imam concludes the prayer, he concludes with him, and his prayer becomes sufficient for Maghrib. Then he prays 'Isha after that. Some of the scholars chose this, such as Shaykh al-Islam Ibn Taymiyyah and a group of scholars; because he is excused in this situation.
It is established from him (peace and blessings be upon him) that he granted a dispensation to the praying people in the prayer of fear due to some excuses in order to preserve the congregation. He prayed, may the prayers and peace be upon him, in the prayer of fear at some times, with the entire army, and he made them two rows, and he recited for them, and bowed with them. Then when he prostrated, he ordered the second row to stay behind in guard so that the enemy would not notice them. So the second row remained standing, watching so that the enemy would not advance and take advantage of their prostration. When the first row stood up from their prostration, the second row descended and prostrated. Then in the second rak'ah, the second row advanced and became the first row, and the first row delayed and became the second row. So they all bowed. When they prostrated, the first row, which had been the second row in the first rak'ah, prostrated with him, and the second row remained guarding. When the first row stood up from prostration, the second row prostrated. This is a deviation from the rule of the prayer of safety, but it was permitted for the sake of the legitimate interest.
Likewise, in the prayer of fear, he would pray with them in two groups: one group guarding, and one group praying with him. When he prayed the first rak'ah with them, they stood up for themselves, separating for themselves in place of the second rak'ah, then they concluded the prayer, then they went to guard. The Prophet, may the prayers and peace be upon him, was standing in the second rak'ah. Then the other group would come and line up with him, and pray the second rak'ah with him. Then they would stand up and perform the second rak'ah that belongs to them, then they would recite the tashahhud, then he, may the prayers and peace be upon him, would conclude the prayer with them. So this is an excuse due to fear, and due to the issue of the guards for the Muslims. So this matter was permitted for the legitimate excuse that occurred due to the fear of the enemies.
So likewise, here, the one who missed Maghrib and came while they are praying 'Isha is excused; because he did not catch up with them before the prayer. And he made a mistake in his delay if he had no excuse. And it may be that he was excused by sleep, or something similar.
So the conclusion: That if he joins them, and prays with them with the intention of Maghrib, and sits in the third rak'ah waiting, then concludes the prayer with the imam, his prayer suffices for him. Then after that, he prays the 'Isha prayer alone, or with those who are available who want to pray voluntary prayers with him, and complete his prayer, so they pray with him until he has a congregation, or with those who have not prayed 'Isha, they pray with him.
It is established from him (peace and blessings be upon him) that he saw a man who entered after 'Asr, having missed the prayer, and no one was with him, so he said: He called them to pray with him so that he would have a congregation. The one who prays with a voluntary prayer, while he is praying an obligatory prayer, this indicates the great status of the congregation, and that it is intended, and that attending it is very important, as mentioned before. For this reason, some excuses are permitted in it due to the desire to attend the congregation and perform the prayer in congregation.