What should one do who, after the Takbir of a supererogatory prayer, remembers that he owes a prostration of forgetfulness (Sujud al-Sahw) from the Maghrib prayer?

Scholars have differed concerning what is required of someone who forgot the prostration of forgetfulness until a long time had passed. As for someone who forgot the prostration of forgetfulness until he entered into another prayer, he should perform it after finishing that prayer, because the prostration remains an obligation upon him, and it is not permissible for him to perform it during his prayer lest he add to it. Nor should he cut short his prayer, due to the Almighty's saying: "And do not invalidate your deeds."