First sitting: a wajib after the first two rakahs of a four-rakah fard prayer. If forgotten and one approaches standing, do not return; use sujud al-sahw.
Final sitting: an essential pillar after the last fard rakah. Prostrating in an extra fifth without completing it causes loss of the fard status.
This distinction underlies the different fifth-rakah cases.
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Prostrations of Forgetfulness: Causes, Method, Missed Sittings and Doubts
First sitting: a wajib after the first two rakahs of a four-rakah fard prayer. If forgotten and one approaches standing, do not return; use sujud al-sahw.
Final sitting: an essential pillar after the last fard rakah. Prostrating in an extra fifth without completing it causes loss of the fard status.
This distinction underlies the different fifth-rakah cases.