What should the worshipper do if his imam stands for the third rak'ah before completing the middle tashahhud, according to the opinion of those who hold that the worshipper should bow with his imam even if he has not completed Al-Fatiha?

Those who obligate the recitation of Al-Fatihah for the congregant (ma'mum) have differed concerning the situation where the Imam bows before the congregant completes Al-Fatihah. The preponderant opinion is that the congregant should complete it, and his delay does not harm him. As for the Imam standing for the third raka'ah before the congregant completes the first tashahhud (definition), the preponderant view among the Shafi'is is that the congregant should complete it due to the slight delay. And if the Imam bows before the congregant completes Al-Fatihah, the preponderant opinion is that the congregant is excused in completing Al-Fatihah.