A man prays Maghrib behind those who are praying 'Isha'; what should he do when they stand for the fourth rak'ah?

The questioner : All right, my Shaykh, regarding a question about prayer: a person entered the mosque intending to pray. It was winter, and the congregation outside intended to combine the prayers. They had finished Maghrib. This latecomer wanted to pray, and they were still intending to pray 'Isha as a combined advance prayer. When he asked them, they said, "We are praying 'Isha as a combined advance prayer." So, does he intend to pray Maghrib, or not?

The Shaykh : Correct.

The questioner : All right, when the imam is about to stand for the fourth rak'ah, what should the one who is in the third rak'ah do?

The Shaykh : He should intend to separate from the imam and complete his prayer with the final tashahhud, and then say the taslim. Yes.

The questioner : And he says the taslim, stands up, and prays 'Isha with them as a combined advance prayer?

The Shaykh : Yes, exactly.

The questioner : Yes, that is correct. May Allah reward you with good, if Allah wills.

The Shaykh : And you, and peace be upon you.
Abu Layla : Is Shaykh Ibn 'Asakir coming to you?

The Shaykh : Yes.Abu Layla : May it be blessed, if Allah wills.

The Shaykh : May Allah bless you.