What is the ruling on congregational prayer if the Imam made a mistake in the final Tashahhud and stood up, adding two rak'ahs, then performed the prostration of forgetfulness (sujud al-sahw)? And is the prayer of the congregant who followed the Imam valid?

If the Imam stands up for a fifth raka'ah mistakenly and completes it after realizing his error, his prayer becomes void. The same applies to the follower (ma'mum) if he knowingly follows the Imam in this excess. However, according to the Hanafis, if the Imam stands up for a fifth raka'ah and does not sit for the tashahhud in the fourth, his obligatory prayer becomes invalid and turns into a supererogatory one. It is then recommended for him to make the fifth raka'ah even with a sixth, and he is obligated to repeat the obligatory prayer. This is the view of Abu Hanifa and Abu Yusuf, whereas Muhammad holds that it becomes absolutely void. But if the Imam had sat for the tashahhud and then mistakenly stood up for a fifth raka'ah, his prayer does not become void, and he should make the fifth raka'ah even with a sixth. If he does not do so, there is no harm upon him.