What is the ruling on the prayer of one who intentionally separates from the Imam without a reason, whether he completely قطع his prayer or completed it alone?

Whoever cuts off a fard (obligatory) prayer after having started it with the congregation, without a valid excuse, has committed two wrongs: First: Cutting off an obligatory prayer without an excuse, which is Haram (forbidden) by consensus. Second: Abandoning the congregation, which is obligatory according to the stronger opinion.

As for his prayer, if he completed it alone after intending to break away from the imam (i.e., ending his followership), there is a difference of opinion on the matter: 1. Those who consider the congregation a condition for the validity of the prayer: His prayer is invalid. This is the view of Ibn Hazm and some Hanbalis. 2. Those who do not consider the congregation a condition: If the cutting off was for an excuse: The majority of scholars hold it to be permissible and valid, and this is the most correct view among the Shafi'is. If the cutting off was without an excuse: Opinions differ: The prayer becomes invalid: This is the relied-upon view among the Hanbalis, and one opinion among the Shafi'is. The prayer remains valid: Ibn Qudamah inclines towards this view, and it is the most correct view among the Shafi'is (who permit breaking away from the imam whether for an excuse or without one).

As for someone who broke away from the imam but did not intend to do so (i.e., did not intend to pray alone), his prayer becomes void by consensus.