What should a person do if he forgets to offer the second sajda [prostration] during congregational prayer and remembers it before the end of the prayer?

Whoever forgets a pillar of prayer while praying in a group must follow the lead of the imam. After the imam makes salams [closes the prayer], he should offer one rak’a [cycle of prayer] and then make sujud as-sahw [prostration of forgetfulness].
And God the Almighty knows best.