Is Sujud Al-Sahw (prostration of forgetfulness) obligatory when in doubt about the number of prostrations during prayer?

Doubt about an increase in the number of prostrations (sujud) is not to be heeded if it occurs after completing the raka'ah or after completing the prayer, because the default is that there was no increase. However, if the doubt about an increase occurs during the prostration itself, then the prostration of forgetfulness (sujud al-sahw) is prescribed.