The World Federation One Stop Fiqh

Ruling 1458

If the imam mistakenly performs qunūt in a rakʿah that does not have qunūt, or if he mistakenly starts saying tashahhud in a rakʿah that does not have tashahhud, then the follower must not perform qunūt or say tashahhud. However, he cannot go into rukūʿ before the imam or stand up before the imam; rather, he must wait until the qunūt and tashahhud of the imam finish and then complete the rest of the prayer with him.