Prayer: Inadvertently Going into Rukuʿ Before the Imam
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1456
If a person inadvertently goes into rukūʿ before the imam, in the event that after saying the obligatory dhikr of rukūʿ he can go back and join part of the imam’s rukūʿ, he must say the dhikr and then, based on obligatory precaution, he must go back into rukūʿ. And the recommended precaution is that he say dhikr in the second rukūʿ as well. If he intentionally does not go back into rukūʿ, the validity of his congregational prayer is problematic [i.e. based on obligatory precaution, it is not valid]. However, his prayer [performed on his own] is valid as per the details mentioned in Ruling 1403. If he cannot go back to say the obligatory dhikr and join the rukūʿ of the imam, he must say the dhikr and then go into sajdah with the imam; if he does this, his congregational prayer is valid.