Ruling on forgetting a rak'ah or more and remembering it after the prayer

His second question says: I prayed with a congregation, but I missed the first rak'ah. When the imam said the taslim, I said it with him. Then I remembered that I still had a rak'ah remaining before I spoke to anyone. Should I repeat the prayer, or should I complete the prayer I started with the imam?
Answer: If a person misses some of the rak'ahs and then says the taslim with the imam while forgetting, he must stand up and complete the prayer. He does not repeat the prayer; rather, he completes what he missed, whether it is one rak'ah or more. Then he performs the prostration for forgetfulness either before the taslim or after the taslim. After the taslim is better because he said the taslim while deficient. His prayer is sufficient and valid, praise be to Allah. If he says the taslim while missing one rak'ah or more with his imam while forgetting, and he is one who joined the prayer late, then when he remembers, he stands and performs the rak'ah or rak'ahs he missed. He performs the prostration for forgetfulness after the taslim. He completes the prayer, then says the taslim, then performs two prostrations for forgetfulness after the taslim. If he performs them before the taslim, there is no harm. This is if he remembers soon. However, if the interval is long and his remembrance is delayed after saying the taslim with his imam, he must repeat the entire prayer. For example, if he said the taslim and forgot until the interval became long, such as saying the taslim for the midday prayer and not remembering until the afternoon prayer or at night, he must make up the entire prayer, meaning he repeats it from the beginning. But if he remembers immediately in his place, or shortly after standing up, taking a few steps, or something similar, it is sufficient for him to repeat the rak'ah, meaning to make up the rak'ah. This is because the Prophet (peace and blessings be upon him) said the taslim from some of the prayers, the afternoon or midday prayer, and stood up, walked, and entered some of his wives' quarters. Then Dhul-Yadayn alerted him, so he returned and completed the prayer without repeating it. Therefore, whoever does what the Prophet (peace and blessings be upon him) did does not have to repeat the prayer; rather, he only performs what he missed, meaning he performs the rak'ah or two rak'ahs he missed. When he says the taslim, he performs the prostration for forgetfulness as the Prophet (peace and blessings be upon him) did. However, if he prolongs the interval significantly, meaning, for example, a time period passes, such as missing it in the midday prayer and not remembering until the afternoon prayer, or missing it in the afternoon prayer and not remembering until the sunset or night prayer, in such a case he must repeat the entire prayer, meaning he repeats it from the beginning, because the interval was long. Yes.