Must the prayer that was performed in congregation be repeated if the worshipper (ma'mum) pronounced the takbirat al-ihram before the imam by mistake?

If a worshiper begins prayer alone and then intends to lead the prayer after someone joins him, his prayer is valid, and this is the most preponderant view. However, if he begins prayer alone with the intention of leading, some jurists hold that his prayer is valid if he expects someone to join him, but invalid if he does not expect anyone. In the event of its invalidity, it must be repeated immediately.

As for a person who begins prayer alone and then intends to join a congregation, there is a difference of opinion regarding this: - One view is that his prayer is invalidated due to the division of intention. - Another view is that his prayer is valid, and this is the correct view according to Sheikh Ibn Uthaymeen, because the change here is in the description of the intention, not its essence. He supported this by analogy with the transition of a solitary worshiper to leading the prayer.