Doubting Impurity After Certainty of Purity, or the Reverse
One who was certain of purity and then doubted whether he broke it, or was certain of impurity and then doubted whether he had purified — which does he act upon?
He acts upon what he was certain of: one certain of purity who doubts impurity is pure, and one certain of impurity who doubts purity is impure, since certainty is not removed by doubt.
One certain of purity who doubts impurity outside the prayer must perform ablution, except one beset by persistent doubt (mustankiḥ). If the doubt arises during the prayer, he must continue it; if his purity then becomes clear he does not repeat it, but if his doubt persists he repeats it, his ablution being void. One certain of impurity who doubts purity must perform ablution.
He acts upon certainty and pays no heed to doubt, on the strength of his saying ﷺ: “Let him not leave until he hears a sound or smells an odour”; one certain of purity who doubts impurity is pure, and one certain of impurity who doubts purity is impure.
He acts upon certainty and sets doubt aside: one certain of purity who doubts impurity acts upon purity and his prayer is valid, and one certain of impurity who doubts purity acts upon impurity and must perform ablution.