A man came to the mosque to pray Maghrib, but he found the people praying, so he prayed with them without knowing that they were praying 'Isha'.

The questioner : 3: A man came to the mosque to pray Maghrib, but he found the people praying 'Isha, though he did not know this, so he prayed with them. When the imam stood for the fourth rak'ah, he realized that it was 'Isha. What should he do? Should he remain sitting until he performs a fourth rak'ah, or should he stand with the imam?

The Shaykh : No, he should separate from the imam and say the taslim. Because he is... praying Maghrib, and Maghrib is not valid if prayed for more than three rak'ahs. So he remains sitting, intending to separate from and part with the imam, and says the taslim from the Maghrib prayer. Then he stands and intends to pray 'Isha behind the imam where he caught up with him. This is how he should do it, and we know of no other way.