What is the ruling on the prayer of one who completed his prayer with a prostration (sajdah) alongside the Imam, who added a fifth rak'ah, thinking he had forgotten a rak'ah, knowing that the Imam had already begun the tashahhud before being alerted?

If the Imam forgot a prostration (sajdah) in the last tashahhud, he should perform it, then recite the tashahhud, make the tasleem, and then perform the prostrations of forgetfulness (sujood as-sahw). It is not permissible to follow him if he stands for an extra rak'ah. Rather, the congregants must glorify Allah (say "Subhanallah") to alert him. If he does not return, they are given the choice between separating from him and completing their prayer individually, or waiting for him to prostrate and make the tasleem with him. Whoever stands with him out of ignorance, their prayer is valid. Waiting for the Imam is better for the congregant, but following him into a fifth rak'ah is not permissible, even if the Imam stands due to forgetting a previous pillar, because the congregant would have already completed their prayer.