If one wants to combine the Maghrib and 'Isha' prayers and finds the imam praying 'Isha'
Machine translationRead the Arabic original
Question:
May Allah reward you and bless you. Abu Nasir says: A man wants to combine the Maghrib and 'Isha' prayers, so he entered the mosque to pray, and he intended to pray Maghrib, but he found the imam praying 'Isha'. What should he do?
Answer:
He may join the imam in the 'Isha' prayer, but when the imam stands for the fourth rak'ah, he should sit and recite the final tashahhud. Then, if he wishes, he may conclude the prayer on his own, or if he wishes, he may wait for the imam and conclude the prayer with him. Yes.
May Allah reward you and bless you. Abu Nasir says: A man wants to combine the Maghrib and 'Isha' prayers, so he entered the mosque to pray, and he intended to pray Maghrib, but he found the imam praying 'Isha'. What should he do?
Answer:
He may join the imam in the 'Isha' prayer, but when the imam stands for the fourth rak'ah, he should sit and recite the final tashahhud. Then, if he wishes, he may conclude the prayer on his own, or if he wishes, he may wait for the imam and conclude the prayer with him. Yes.
Source
نور على الدرب - الفوزان - 03/173 — archive.org (9:45)
The Arabic text is a machine transcription of the audio recording.