What should a worshipper who has forgotten a pillar of prayer do?

A person who remembers that he has forgotten a pillar of prayer before offering it in the following rak’a [cycle of prayer], should offer it and continue the prayer. He should then make sujud as-sahw [prostration of forgetfulness] at the end of the prayer. But if he remembers that he has forgotten a pillar after having offered it in the following rak’a, then he should offer a new rak’a in its place and make sujud as-sahw .
And God the Almighty knows best.