This involves two possibilities: One is that it is not incumbent upon him, because the original state [of the mouth] is purity, so that [status] does not cease due to doubt. The second is that it is incumbent upon him, because it is a place where its use was forbidden due to impurity, so he is required to wash its trace, like [a case of] certain impurity.

Section: If he identifies the specific impure item, it is recommended to pour it out to remove the doubt from himself. If he needs it for drinking, he drinks from the pure one, and performs Tayammum if he finds nothing but the impure. If he fears thirst for the second [scenario, i.e., later], the Qadi said: He performs Wudu with the pure [water] and retains the impure one, because he is [not in need] of drinking it at the moment, so performing Tayammum while it is present is not permissible.

The correct view, God willing, is that he [pours out the impure] and performs Tayammum, because the presence of the impure water is like its absence when there is a need for drinking immediately, and likewise in the future; and fear of thirst regarding the permissibility of Tayammum is like the reality of it.

Section: If pure water is indistinguishable from water whose purity has been invalidated, he performs Wudu completely from each of them, and prays one prayer with both Wudu acts. I know of no disagreement regarding this; because it is possible for him to perform his mandatory obligation with certainty without any hardship, so it is incumbent upon him, just as if they were both pure and one of them did not suffice. It differs from the case where it is impure, because it would make his limbs impure with certainty, and he is not safe from the possibility that the second one is the impure one, whereby he would remain impure and his prayer would not be valid. If he needs one of the two containers for drinking, he practices Ijtihad and performs Wudu with the one he deems pure, and performs Tayammum along with it to achieve certainty. And God knows best.

Section: If he cannot distinguish between pure garments and impure ones, Ijtihad is not permissible. He must pray in each garment according to the number of impure items, plus one additional prayer. This is the opinion of Ibn al-Majishun.