The World Federation One Stop Fiqh

Ruling 1423

With regard to someone who knows that if he completes the other surah or qunūt he will not be able to join the imam in rukūʿ, in the event that he intentionally recites the other surah or performs qunūt and does not join the imam in rukūʿ, his congregational prayer becomes invalid and he must act according to the duty of one who performs prayers on his own.