In the performance of obligatory prayers (farz namaz), the first sitting (tahiyyat) after the second rak’ah is considered wajib (necessary). If this first sitting is accidentally omitted, it does not invalidate the prayer, but the person must perform the prostration of forgetfulness (sehiv sajdah) during the final sitting to compensate for this omission.
Ruling on Deliberate Omission
Deliberately skipping the first sitting without a valid reason is considered tahrimen makruh (prohibitively disliked). If a person intentionally neglects this act, it is recommended to repeat the prayer to ensure its validity and completeness (Ibn Nujaym, al-Bahr, 1/310; Ibn Abidin, Radd al-Muhtar, 2/88).
Summary
Accidental Omission: If forgotten, perform sehiv sajdah during the last sitting.
Deliberate Omission: It is strongly disliked and requires the prayer to be repeated.
By adhering to these guidelines, one ensures that the prayer is performed correctly according to Islamic teachings.