What is the ruling regarding the follower (ma'mum) continuing with the imam if the imam stands for a fifth rak'ah inadvertently in one of the 'Isha prayers? Should he wait for him while sitting, then say the tashahhud and taslim with him? And should the imam sit down if he realizes his mistake, even if he has already stood up straight?

If the Imam stands up for an extra (fifth) unit of prayer (rak'ah), the congregant (ma'mum) must alert him. If the Imam does not return, it is not permissible for the congregant to follow him. Instead, he should sit until the Imam finishes and make the salutation (taslim) with him. Whoever follows the Imam knowing about the extra rak'ah, their prayer is invalid, and they must repeat it. However, if one does not realize the extra rak'ah until after the salutation, their prayer is valid. The Imam must sit down if he realizes his mistake or if the congregants alert him. If he completes the extra rak'ah while knowing it is an addition, his prayer is invalid, and he must repeat it.