If the imam sits for the tashahhud in the first rak'ah by mistake, he must perform the prostration for forgetfulness
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The second question of fatwa number ( 18573 )
Q2: An imam prayed the first rak'ah and sat for the tashahhud, but then he realized he was in the first rak'ah. He then stood up to complete the prayer and performed the prostration for forgetfulness at the end. What is the ruling on that?
A2: If the imam sits for the tashahhud in the first rak'ah by mistake, it is obligatory for him to perform the prostration for forgetfulness before or after the taslim; because he added to the actions of the prayer. And success is from Allah, and may Allah send prayers and blessings upon our Prophet Muhammad, his family, and his Companions.
Q2: An imam prayed the first rak'ah and sat for the tashahhud, but then he realized he was in the first rak'ah. He then stood up to complete the prayer and performed the prostration for forgetfulness at the end. What is the ruling on that?
A2: If the imam sits for the tashahhud in the first rak'ah by mistake, it is obligatory for him to perform the prostration for forgetfulness before or after the taslim; because he added to the actions of the prayer. 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.