What is required of the ma'mum if the imam stands for an extra rak'ah?
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Question:
This is a question about when the imam makes a mistake and stands for an extra rak'ah, and the ma'mum knows that it is extra. What does he do?
Answer:
It is obligatory for him not to follow; he should sit down and not follow him in the extra rak'ah. However, he must alert the imam by saying, "Subhan Allah." He must alert the imam. If the imam does not return, he does not follow him; he sits until the imam returns and says the taslim with him, and he performs sujud al-sahw with him. Yes.
This is a question about when the imam makes a mistake and stands for an extra rak'ah, and the ma'mum knows that it is extra. What does he do?
Answer:
It is obligatory for him not to follow; he should sit down and not follow him in the extra rak'ah. However, he must alert the imam by saying, "Subhan Allah." He must alert the imam. If the imam does not return, he does not follow him; he sits until the imam returns and says the taslim with him, and he performs sujud al-sahw with him. Yes.
Source
قناة «العلامة الدكتور صالح الفوزان»، ماذا على المأموم إذا قام الإمام لركعة زائدة؟ للشيخ الفوزان — youtube.com
The Arabic text is a machine transcription of the audio recording.