While praying Zuhr, I thought I was in the 3rd rakat, so I didn’t read another Surah after the Fatiha. It was actually the 2nd rakat. Do I have to make Sajda al-Sahw? What if I forgot to make Sajda al-Sahw?
May Allah bless you for your concern for the prayer.
Reciting the Fatiha and another Surah or three verses is necessary (wajib), so forgetting to recite the equivalent of three verses would require the prostration of forgetfulness (sajda al-sahw). (Mawsili, Ikhtiyar) Your recital of the Fatiha fulfills the necessary (wajib) amount, so there is no need to make the prostration of forgetfulness (sajda al-sahw).
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And Allah knows best.
[Ustadh] Nail Kadi