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

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