What is the ruling on the prayer of one who prayed behind an imam as a follower, without the intention of following the imam due to ignorance, and did not specify the prayer before it or with the takbīrat al-iḥrām? And is merely going to the mosque and standing in the صف (row) sufficient as an intention for prayer?

The question includes two issues:

First: The intention of the worshipper to follow the imam, which is obligatory. As for the prayers performed without this intention, there is a difference of opinion regarding them. The madhhab of Shaykh al-Islam Ibn Taymiyyah and those who agreed with him is that there is no need for re-performing the prayer for someone who missed a condition or a pillar out of ignorance. It is permissible to follow this opinion, especially if re-performing the prayer involves hardship.

Second: Intention is a pillar of prayer, and it is best for it to be simultaneous with the takbīrat al-iḥrām. It is permissible for it to precede it by a short time. There is an opinion held by some scholars that going to the mosque or standing in the row is sufficient for the intention of prayer, and that a significant lapse of time between the intention and the prayer does not harm as long as the intention is not revoked. This opinion contradicts the madhhab of the majority, but many scholars have permitted issuing a fatwa based on it after the action has occurred and when rectifying it would involve hardship.