If the imam forgets a prostration and does not perform it, it is obligatory for the one praying behind him to perform a complete unit of prayer
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Fatwa number ( 17456 )
Q1: A man prayed the midday prayer in congregation, and in one of the units the imam forgot a prostration , and someone behind him reminded him. After the final tashahhud, the imam prostrated twice for forgetfulness, and no one objected to that. So, is it required for the one praying behind him to follow the imam in the taslim and then perform a unit and prostrate for forgetfulness, or should he not say the taslim, perform another unit, and prostrate for forgetfulness, or is prostration for forgetfulness not required of him? When is the prostration for forgetfulness performed, if it is required of him, before the taslim or after it? Please inform us, may Allah reward you with good.
A1: It is required for the one praying behind him who knows about the omitted prostration not to say the taslim with the aforementioned imam. He must perform a unit in place of the unit from which the prostration was omitted, if the prostration was in a unit other than the last one, then he performs the final tashahhud and then says the taslim. If the prostration was in the last unit, he performs the omitted prostration before the taslim, then performs the tashahhud and says the taslim. If the time interval becomes long, he must repeat the aforementioned prayer.
Q1: A man prayed the midday prayer in congregation, and in one of the units the imam forgot a prostration , and someone behind him reminded him. After the final tashahhud, the imam prostrated twice for forgetfulness, and no one objected to that. So, is it required for the one praying behind him to follow the imam in the taslim and then perform a unit and prostrate for forgetfulness, or should he not say the taslim, perform another unit, and prostrate for forgetfulness, or is prostration for forgetfulness not required of him? When is the prostration for forgetfulness performed, if it is required of him, before the taslim or after it? Please inform us, may Allah reward you with good.
A1: It is required for the one praying behind him who knows about the omitted prostration not to say the taslim with the aforementioned imam. He must perform a unit in place of the unit from which the prostration was omitted, if the prostration was in a unit other than the last one, then he performs the final tashahhud and then says the taslim. If the prostration was in the last unit, he performs the omitted prostration before the taslim, then performs the tashahhud and says the taslim. If the time interval becomes long, he must repeat the aforementioned prayer.
Source
www.alifta.gov.sa
The Arabic text is copied verbatim from the original source, without any edits.