He forgot the second prostration in the second unit of the Fajr prayer, then stood up and performed a unit
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Q2: I prayed behind an imam in the Fajr prayer. During the sitting between the two prostrations in the second unit, he forgot and made it the final sitting for the tashahhud. After that, he concluded the prayer with the taslim. The worshippers informed him that there was a deficiency in his prayer, so he stood up and said the takbir to repeat the unit. I thought he would repeat the entire prayer, so I said the takbir with the intention of repeating the prayer. He prayed one unit, in which he performed the prostration for forgetfulness, and then concluded the prayer with the taslim. I concluded the prayer behind him, saying, "I will do what the imam does." Please rule on this for me?
A2: If one omits the final prostration of the prayer and concludes it with the taslim, he must return and perform the prostration, then perform the final tashahhud after it, and perform the prostration for forgetfulness. He does not perform a unit. What this imam did is considered an error and an addition to the prayer, but he is excused due to ignorance. Your intention to repeat the prayer, based on your assumption that the imam would repeat it, does not affect the validity of your prayer. And success is from Allah, and may Allah send prayers and blessings upon our Prophet Muhammad, his family, and his Companions.
A2: If one omits the final prostration of the prayer and concludes it with the taslim, he must return and perform the prostration, then perform the final tashahhud after it, and perform the prostration for forgetfulness. He does not perform a unit. What this imam did is considered an error and an addition to the prayer, but he is excused due to ignorance. Your intention to repeat the prayer, based on your assumption that the imam would repeat it, does not affect the validity of your 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.