What must the follower do if the Imam forgetfully adds a third prostration (Sajdah) in a rak'ah?

Excuses for Missing Congregation and Selected Rulings for Followers

• The follower must perform the third prostration with him.
• The follower must NOT follow him in this invalid addition and must remain waiting.
• The follower must immediately perform the Salaam on his own.
• The follower must shout out "Allah-u-Akbar" to stop him.