The ma'mum's position when the imam adds or omits a rak'ah

The questioner:
The questioner says: If the imam forgets and is reminded to come for a rak'ah, but he does not comply, what is the ma'mum's position?

The Shaykh:
Coming for a rak'ah means an extra one, or what?

The questioner:
It might be a missing one, and Allah knows best.

The Shaykh:
He was reminded of a missing rak'ah.

The questioner:
The question was not clarified.

The Shaykh:
If the ma'mum is certain that it is an extra rak'ah, he does not follow the imam; he sits down and does not follow him, knowing that it is extra. Or if he is certain that it is a missing one, and the prayer has ended and the imam has said the taslim, he does not say the taslim with him; rather, he stands up and performs the missing rak'ah. Yes.