What should the worshippers do if the Imam sits for the final Tashahhud in the third Rak'ah of the 'Isha prayer?

The congregants should alert the Imam by saying "Subhanallah" (Glory be to Allah). If he does not understand, they should speak to him to make him understand. If he complies, they complete the remaining rak'ah with him and perform the prostrations of forgetfulness (sujud al-sahw) after the salām. If he does not comply, the congregants complete the remaining rak'ah individually, or they may put one of them forward to lead them as their Imam. The Imam's prayer is valid if he realizes his mistake and performs the remaining rak'ah before a long separation of time.