If the imam errs and omits the fourth rak'ah, and is reminded but does not perform it, it is obligatory for the ma'mums to perform it
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Fatwa number ( 16203 )
Q: The imam of the mosque prayed with us in a four-rak'ah prayer, and we were behind him. He sat for the first sitting, then added one rak'ah and sat. The ma'mums recited the tasbih, but he did not notice, and he said the taslim. Are the congregation obligated to complete the fourth rak'ah after him, or should they follow the imam and repeat the prayer?
A: Both the imam and the ma'mum are obligated to perform the fourth rak'ah to complete the prayer. The imam must stand to perform the fourth rak'ah by facing the qiblah while sitting, then standing up to perform the rak'ah. The ma'mums must follow him and not separate from him. Then he prostrates with them the two prostrations of forgetfulness. If the imam does not stand up after they remind him, they stand up and complete their prayer, then say the taslim, and they owe nothing. And success is from Allah, and may Allah send prayers and blessings upon our Prophet Muhammad, his family, and his Companions.
Q: The imam of the mosque prayed with us in a four-rak'ah prayer, and we were behind him. He sat for the first sitting, then added one rak'ah and sat. The ma'mums recited the tasbih, but he did not notice, and he said the taslim. Are the congregation obligated to complete the fourth rak'ah after him, or should they follow the imam and repeat the prayer?
A: Both the imam and the ma'mum are obligated to perform the fourth rak'ah to complete the prayer. The imam must stand to perform the fourth rak'ah by facing the qiblah while sitting, then standing up to perform the rak'ah. The ma'mums must follow him and not separate from him. Then he prostrates with them the two prostrations of forgetfulness. If the imam does not stand up after they remind him, they stand up and complete their prayer, then say the taslim, and they owe nothing. 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.