What is the ruling on following the Imam if he adds an extra Rak'ah to the prayer?

If one prays behind an Imam who makes a mistake and stands for an extra rak'ah, it is not permissible for the ma'mum (follower) to follow him. If one follows him knowingly, their prayer is invalidated. The ma'mum has the choice between waiting for the Imam to conclude the prayer with him, or separating from him and concluding the prayer on their own. As for one who stood up out of ignorance, if they realize their mistake before the prayer time expires, they must repeat the prayer. But if they realize it after the prayer time has expired, then there is nothing upon them, and the hadith (definition) of the one who prayed badly is used as evidence for this.