The imam made a mistake and added a fifth rak'ah, and the ma'mumin reminded him, but he did not sit, so they differed regarding him
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Fatwa number ( 17158 )
Q: While we were praying the 'Asr prayer one day, the imam prayed four rak'ahs, then stood up after the fourth and added a fifth. The ma'mumin reminded him, saying: "Subhan Allah," so that he would sit, because he had fully stood up, but he did not sit and added a fifth. Then he prostrated twice for forgetfulness before the taslim, then he said the taslim from the prayer. After that, the people differed; some said the prayer is invalid, and some said it is valid. The imam said: "It is not valid to return to sitting for the final tashahhud after the imam has fully stood up or was closer to standing, and therefore I did not return to the final tashahhud for that reason, just as it is permissible in the first tashahhud." We ask you to answer this question, and may Allah reward you with good?
A: If one stands up to a fifth rak'ah in the obligatory prayer while forgetting, then remembers
or is reminded by the congregation that it is an extra one, it is obligatory for him to return, whether he has fully stood up or not. If he continues in the fifth while knowing that it is the fifth and knowing that he is obligated to return, his prayer is invalid. Likewise, the prayer of one who follows him while knowing that it is an extra one and that he is obligated to return is invalid. But if one is ignorant of the ruling, his prayer is valid, because he is excused by ignorance. However, it is incumbent upon you in the future to learn this ruling and act upon it. And success is from Allah, and may Allah send prayers and blessings upon our Prophet Muhammad, his family, and his Companions.
Q: While we were praying the 'Asr prayer one day, the imam prayed four rak'ahs, then stood up after the fourth and added a fifth. The ma'mumin reminded him, saying: "Subhan Allah," so that he would sit, because he had fully stood up, but he did not sit and added a fifth. Then he prostrated twice for forgetfulness before the taslim, then he said the taslim from the prayer. After that, the people differed; some said the prayer is invalid, and some said it is valid. The imam said: "It is not valid to return to sitting for the final tashahhud after the imam has fully stood up or was closer to standing, and therefore I did not return to the final tashahhud for that reason, just as it is permissible in the first tashahhud." We ask you to answer this question, and may Allah reward you with good?
A: If one stands up to a fifth rak'ah in the obligatory prayer while forgetting, then remembers
or is reminded by the congregation that it is an extra one, it is obligatory for him to return, whether he has fully stood up or not. If he continues in the fifth while knowing that it is the fifth and knowing that he is obligated to return, his prayer is invalid. Likewise, the prayer of one who follows him while knowing that it is an extra one and that he is obligated to return is invalid. But if one is ignorant of the ruling, his prayer is valid, because he is excused by ignorance. However, it is incumbent upon you in the future to learn this ruling and act upon it. And success is from Allah, and may Allah send prayers and blessings upon our Prophet Muhammad, his family, and his Companions.
Source
www.alifta.gov.sa
The Arabic text is copied verbatim from the original source, without any edits.