What is the ruling if a person sits down in place of standing immediately after the first or third Rak'ah, believing it to be the sitting position for Tashahhud or separation?

Primary text

If the person realizes the mistake and rises immediately, their prayer is sound. If they remain seated until they fully rise (i.e., complete the standing posture), they must complete their prayer and perform the prostrations of forgetfulness (Sujud as-Sahw). This is because adding a component of the prayer that would invalidate it if done intentionally necessitates prostration for forgetfulness when done unintentionally, similar to adding an extra Rak'ah.