Should the Imam return to a sitting position after standing upright in the last rak'ah of Maghrib prayer, upon being alerted by the congregants, or should he continue his prayer and perform the prostration of forgetfulness (sujud al-sahw) before the taslim (salutation)?

If the Imam stands for an extra rak'ah mistakenly, he must sit down and then perform the prostrations of forgetfulness (sujud al-sahw). If he does not sit down, his prayer becomes invalid if he was aware and deliberate. However, if he stands for a reason (such as omitting Al-Fatiha), then he completes that rak'ah, and his prayer is valid. It is not permissible for the congregant to follow him in either case.