What should the congregation do if the imam stands up without performing the final sitting?

If the imam stands up without performing the final sitting due to forgetfulness, the congregation should not stand up with the imam but remain seated and wait for the imam to return. In the meantime, “subhanallah” is said aloud to alert the imam. As long as the imam has not performed the prostration of the rak‘ah he stood up for, those following him cannot exit the prayer by giving salam, since there is still a possibility that the imam might return. If they do give salam, their prayer becomes invalid. If the imam does not return and continues the prayer without realizing his mistake and performs the prostration of the rak‘ah he stood up for, then both the imam’s and the congregation’s prayers become invalid for omitting the final sitting.

If the congregation stands up with the imam and the imam realizes his mistake and sits back down before performing the prostration, the congregation should also sit down. If the congregation does not notice that the imam has sat down and proceeds to prostration, this does not invalidate their prayer. In this case, the validity of the imam’s prayer, to which the congregation is following, is what matters. (Ibn Nujaym, al-Bahr, 2/111-112)

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu