Prayer: Being Certain of Joining the Imam in Rukuʿ but Failing to Do So
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1425
With regard to someone who is certain that if he recites the other surah he will be able to join the imam in rukūʿ and he will be able to follow the imam, in the event that he recites the other surah but is unable to join the imam in rukūʿ, his congregational prayer is valid.