We performed Asr Prayer behind the Imam but after the 4th rak’ah he stood and performed a 5th rak’ah, on Muswalli alerted him but he continued. Some of the congregation stood with the Imam and others remained seated. One Muswalli then said: follow the Imam, so some people stood and others remained seated till the Imam made sajdah sahw and made taslim, those who remained seated made talsim with the Imam bearing in mind that some people made taslim before Imam and others left the prayer. When Prayer ended the Imam was asked: why did ypu make the 5th rak’ah and he replied them: it is because I forgot to read surah al-Fatihah on the 2nd rak’ah. At this point some congregants said: Prayer is correct for those eho did not stood up with the Imam and invalid for those who stood with the Imam. And when the person who said: follow the Imam was asked why he did that? He replied by saying it is compulsory to follow the Imam in every situation from the first takbir to when the prayers ends because this is supported by an authentic hadith which instruct that. زاد الإمام ركعة فما الحكم؟

Praise be to Allah, and may Allah’s peace and blessings be upon the Messenger of Allah, his family and his companions.
As to what follows:
With regards to answering your question we say, success is by Allah Almighty.
The scholars differed on what a follower should do if the Imam stands up for an extra rak’ah like 5th rak’ah for Asr Prayer.
Some said he stands up with him, others said he should leave him and make taslim, others went on to say that he sits down and waits for him so that they can make taslim together, others said the follower has an option between waiting for him and leaving him.
What is apparent to me is that if an Imam stands up because of omitting ruku’ or sajda then it is compulsory to follow him because the rak’ah in which some postures were omitted is not adequate. If this is not the case then it is obligatory for the follower to make tasbih for him to come back and sit, if he doesn’t come down to sit then the follower remains seated and does not follow him. The best is for the follower to wait for the Imam so that they can make taslim together. If he makes taslim before the Imam, his Prayer will be coorect.
The followers are not bound by being informed by the Imam after Prayer that he omitted reading surah Fatiha in one of the rak’ats. This is because that shortage / omission done by the imam does not lessen the Prayer of the followers.
And Allah knows best.

Your brother,
Dr. Khalid al-Muslih
20 / 11 / 1424 AH