What is the ruling on someone who mistakenly stood for the third rak'ah during the middle tashahhud, then returned to sit with the imam; does their prayer become void?

If the congregant precedes his Imam in movement (such as standing, bowing, or prostrating), he must return to follow his Imam, and his prayer is not invalidated by his return.

As for the prostration of forgetfulness (sujood as-sahw) for the congregant in this case, it is detailed as follows:

1. If the congregant caught all the rak'ahs with the Imam: There is no prostration of forgetfulness upon him. Scholars have reached a consensus on this, as conveyed by Ibn al-Mundhir. 2. If the congregant joined late (missed rak'ahs) and made a mistake in his prayer: He prostrates for forgetfulness, whether he made the mistake while with the Imam or after he stood up to make up what he missed. His prostration will be after the Imam finishes his prayer.

Example: A congregant forgot to say the glorification (tasbeeh) in bowing, and he did not miss anything [of the prayer], so there is no prostration of forgetfulness upon him. However, if he missed one rak'ah or more, he makes it up and then prostrates for forgetfulness before the salaam. Another example: A congregant thought the Imam had said the salaam, so he sat down and then stood up with him. If he did not miss anything of the prayer, there is no prostration of forgetfulness upon him. But if he missed one rak'ah or more, he makes it up, says the salaam, then prostrates for forgetfulness, and says the salaam.