What is the ruling regarding the prayer of a congregant (ma'mum) who was unable to hear some of the imam's transitional takbirat and doubted the validity of his following (iqtida')? Is it permissible for him to change his intention from following to praying individually and then return to the intention of following? And is he required to repeat those prayers as a precaution?

For the congregant's prayer to be valid when following the imam, he must be aware of the imam's transitions. It is permissible for someone who intended to pray individually due to an excuse to revert to the intention of following the imam once the excuse is gone, and his prayer will be valid. However, if he separates from the imam despite being able to follow him by observing him, then there is an error in that. Scholars have differed regarding the ruling on the prayer of an ignorant person who separated from the imam without an excuse: some have treated him like someone who forgot, so his prayer is valid, while others have treated him like someone who did so intentionally, so his prayer is invalidated, and he is required to repeat it.