In case the Imam forgets and offers a fifth Rak`ah but does not sit down when the Ma'mums remind him

Q: One day while we were performing `Asr (Afternoon) Prayer, the Imam (the one who leads congregational Prayer) stood up for a fifth Rak`ah (unit of Prayer). Hence, the Ma'mums (those being led by an Imam in Prayer) reminded him by reciting Tasbih (saying: "Subhan Allah [Glory be to Allah]"). Nevertheless, the Imam - who stood up fully - did not sit down. Rather, he completed the fifth Rak`ah and then made Sujud-ul-Sahw (Prostration of Forgetfulness) before Taslim. Then the people disagreed. Some said that the concerned Salah (Prayer) was Batil (null and void). Others claimed it was valid, whereupon the Imam said that it is not permissible for a person to return to the sitting position to make Tashahhud (a recitation in the sitting position in the second/ last unit of Prayer) after standing up fully for a fifth Rak`ah. Could you please inform us of your opinion on this? May Allah reward you with the best! A: If a person stands up - fully or partially - for a fifth Rak`ah in an obligatory Salah and remembers (Part No. 6; Page No. 31

A: If a person stands up - fully or partially - for a fifth Rak`ah in an obligatory Salah and remembers (Part No. 6; Page No. 31) or is reminded that such Rak`ah is additional, he has to return to the sitting position. If the person completes the fifth Rak`ah even though he knows that it is an extra one and that he has to return to the sitting position; his Salah is Batil. The same ruling applies to the Salah of the Ma'mums who follow the Imam even though they know that he is performing an extra Rak`ah and that they have to return to the sitting position. It is worth mentioning that the Salah of those who are not aware of this ruling is valid i.e. they are excused because of their ignorance. Any way, you should not make the same mistake in the future.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.