Is the prostration of forgetfulness (Sujud al-Sahw) obligatory when one intentionally invalidates an essential part of the prayer?
Chapter on the Two Prostrations of Forgetfulness
Al-Mughni
Book of Prayer
Primary text
The prostration of forgetfulness is obligatory when an intentional act invalidates the prayer. This is supported by the action and command of the Prophet, peace be upon him, as narrated by Ibn Mas'ud and Abu Sa'id, and based on the command, 'Pray as you have seen me praying.' The statement that the missed unit (rak'ah) or prostrations are optional (nafilah) means they carry reward, similar to how a full rak'ah, which is obligatory upon the forgetful, is termed optional.
Supporting text
There is a dissenting view held by Ahmad (in one account), Shafi'i, and the People of Opinion, suggesting it is not obligatory. Their basis is that the required reparations (Sujud) for non-obligatory acts are not themselves obligatory.