If one omits saying (O my Lord, forgive me), is the prostration for forgetfulness obligatory?

Question : If you omitted saying "O my Lord, forgive me" between the two prostrations,

Answer : then the prostration for forgetfulness is obligatory for you, because if you had intentionally omitted it, your prayer would have been invalid. The prostration for forgetfulness is obligatory in every case where intentionally omitting the act would invalidate the prayer.