The Ruling on Forgetfulness in Prayer
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Question 410: The questioner says: The imam of the mosque led us in the Zuhr prayer, and after the fourth rak'ah he stood up, thinking that he had only prayed three. What is the ruling in this situation? What is the ruling for those who joined him late, and what are the rulings that apply to the ma'mumin (those praying behind the imam)?
A410: If the imam stands up for an extra rak'ah, it is obligatory for the ma'mumin who know that it is extra to alert him by saying, "Subhan Allah" (Glory be to Allah), and they should continue to say this until he returns. If he does not return, then the one who knows that this rak'ah is extra should not stand with him; rather, he should sit, meaning he should wait until the imam says the taslim, and then say the taslim with him. If he wishes, he may say the taslim for himself.
A410: If the imam stands up for an extra rak'ah, it is obligatory for the ma'mumin who know that it is extra to alert him by saying, "Subhan Allah" (Glory be to Allah), and they should continue to say this until he returns. If he does not return, then the one who knows that this rak'ah is extra should not stand with him; rather, he should sit, meaning he should wait until the imam says the taslim, and then say the taslim with him. If he wishes, he may say the taslim for himself.
Source
www.alifta.gov.sa
The Arabic text is copied verbatim from the original source, without any edits.