Is prostration of forgetfulness (Sujud as-Sahw) obligatory if one sits for the duration of the sitting rest (Jalsat al-Istiraha) in a place other than the proper position for the Tashahhud?
Chapter on the Two Prostrations of Forgetfulness
Al-Mughni
Book of Prayer
Primary text
The obligatory requirement for Sujud as-Sahw attaches if one sits for the duration of the sitting rest outside of the Tashahhud position. This is the ruling stated by the Qadi. This obligation remains whether the sitting rest is considered legislatively recommended (Sunnah) or not, because the sitting was not intended for that specific purpose (Jalsat al-Istiraha) but rather for something else, thus it constitutes an act of forgetfulness (Sahu).
Supporting text
It is possible that Sujud as-Sahw is not obligatory because performing that specific action (sitting the duration of Jalsat al-Istiraha) would not invalidate the prayer even if done intentionally. Therefore, one should not perform Sujud as-Sahw for forgetting it, similar to a minor action that is not part of the prayer itself.