A prostration forgotten in the earlier rak‘a performed in the next one

Question

Someone prayed and in the first rak‘a forgot [one] prostration, and in the second rak‘a he made three prostrations. Is his prayer valid? Likewise, if he forgot Surat al-Fatiha, what should he do? If he performs sajda al-sahw, then why? [Please answer] with evidence.

Answer

If in one rak‘a he made only one prostration and forgot the second, and in the second rak‘a made three prostrations, and then also performed sajda al-sahw, the prayer is valid. Because Surat al-Fatiha was forgotten in the first [or] second rak‘a, sajda al-sahw also becomes necessary (lazim)

Source: Fatawa Mahmudiyya

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