Answered by Ustadh Tabraze Azam
Question: Assalamu ‘alaykum,
Should I perform the prostrations of forgetfulness for starting a surah in prayer, forgetting it and reciting another surah instead?
Answer: Wa alaikum assalam wa rahmatullah,
No, you do not need to perform prostrations of forgetfulness (sajda al-sahw) for forgetting the chapter (surah) you were reciting because those prostrations are only required in the case of forgetfully omitting a necessary (wajib) act.
However, if you forget the specific chapter in future, and you have recited three verses or more, you can simply go into the bowing (ruku`) and continue your prayer. You aren’t obligated to complete an entire chapter in the cycle (rak`at).
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And Allah alone knows best.
Checked & Approved by Shaykh Faraz Rabbani.