The imaam forgot and stood up for a third rak’ah in Salaat al-Taraaweeh,then he sat down

If an Imam in Salah Taraweeh forgot to sit down for tashahud in the 2nd rak'ah and get up straight (few seconds) but without utering AlFatiha and then get down to perform the tashahud. What must the Imam and the makmum do?

Praise be to Allah, and blessings and peace be upon the Messenger of Allah:

They should all do sujood al-sahw for this extra action, which was standing up for a third rak’ah.