Forgetting the First Sitting and then Sitting Down to Repeat it

Q: Salaah-22: Will a four Rakaah Fardh or Waajib salaah be nullified by sitting down from Qiyaam (standing posture, which is Fardh) to complete the first sitting (Qa'dahUla, which is Waajib) if one had forgotten to sit?

Answer: If a person realised that he did not sit for the first sitting (Qa'dah Ula) and then sits down from Qiyaam or sits down when closer to the Qiyaam posture, it would not mean that he had left out a Fardh (obligatory) act to perform a Waajib (compulsory) act, but only that he delayed a Waajib (compulsory) act. Sajdah Sahw will therefore remedy the situation and the salaah will not be invalidated. This should however not be done and to do so intentionally will make one a sinner.

Fatawa Rahimiyyah vol.2