What is the ruling if the Imam remembers the first Tashahhud after the followers have risen and begun recitation, and he returns to it?

Primary text

If the Imam remembers the first Tashahhud before he has fully stood up, or after the followers have risen and begun recitation, and he returns to the sitting, the followers must follow him back because the Imam returned to an obligatory act, compelling their obedience. The prior standing of the followers is disregarded in this case.