If the Imam performs the Salaam before the follower has finished reciting the Tashahhud in the sitting posture, what is the follower legally required to do?

He must perform the Salaam immediately with the Imam and skip the rest of the Tashahhud.
He must stand up and repeat the entire rak'ah.
He must complete his Tashahhud first, and then perform the Salaam.
He must perform Sujud al-Sahw.