Should a latecomer who stood for the third rak'ah after the Imam rose from prostration and before the middle tashahhud sit, or should he wait for the Imam to say the taslim?

The worshipper is obliged to follow his Imam in the actions of the prayer, for the Prophet, peace and blessings be upon him, said: "Indeed, the Imam was appointed to be followed." It is not permissible to precede him in any action. If the worshipper stands alone before his Imam, he must return to follow the Imam. Some scholars have differentiated between the intentional and the forgetful; whoever does it forgetfully must return, and whoever does it intentionally is not required to return.