Sajda al-Sahw for Omitting the First Sitting in Sunna and Nafl Prayers

Question

If [a person] forgets the “al-tahiyyat” in the first sitting of a four-rak'a sunna, or within a two-rak'a sunna or nafl, and then in that state sits, performs sajda al-sahw and completes the prayer — is his prayer valid or not?

Answer

In a four-rak'a sunna the first sitting and the tashahhud are wajib, and omitting them necessitates sajda al-sahw. In a nafl prayer, the sitting after two rak'as is fard, and if it is omitted the prayer will not be valid.

Source: Fatawa Mahmudiyya

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