I say, and with God's success: He must strive – that is, exert his effort to know the pure garment from the impure – in all cases, whether most of the garments are pure, or most of them are impure, or they are equal in impurity and purity; because covering the private parts in prayer is obligatory, and there is no substitute for the garment in covering the private parts, unlike striving in water; because water can be replaced by soil. If he prays while striving and then realizes that his judgment was wrong, he must repeat the prayer either within the time or afterward. See: Maraqi al-Falah, pp. 34-35, and God knows best.