The ruling on one who forgets the second prostration until he has said the closing salam
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Question:
I prayed behind the Shaykh (may Allah preserve you) as imam. In the last rak'ah, when did he… the second prostration until he said the salam, and he realized he was in the third [rak'ah]? What should he do? Does he need to perform a complete rak'ah, or just the prostration? And when does he perform the prostration?
Answer:
If he remembers after the salam, provided the interval has not been long, he returns and performs the second prostration. He prostrates the second prostration, then recites the final tashahhud after it, then says the salam, and then prostrates for forgetfulness after the salam. However, if he did not know, did not remember, and did not recall it until the interval had become long, or he had left the mosque, he must repeat the prayer in full. If the interval has become long, or if his wudu' has been broken, he must repeat the prayer in full.
I prayed behind the Shaykh (may Allah preserve you) as imam. In the last rak'ah, when did he… the second prostration until he said the salam, and he realized he was in the third [rak'ah]? What should he do? Does he need to perform a complete rak'ah, or just the prostration? And when does he perform the prostration?
Answer:
If he remembers after the salam, provided the interval has not been long, he returns and performs the second prostration. He prostrates the second prostration, then recites the final tashahhud after it, then says the salam, and then prostrates for forgetfulness after the salam. However, if he did not know, did not remember, and did not recall it until the interval had become long, or he had left the mosque, he must repeat the prayer in full. If the interval has become long, or if his wudu' has been broken, he must repeat the prayer in full.
Source
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The Arabic text is a machine transcription of the audio recording.