פירוש על פרה 5:2

המטביל כלי לחטאת (a person who immerses a vessel for a purification rite) – a vessel which requires only rinsing in order to be restored to Levitical cleanness that he immersed it in order to fill it with water and to mix them with ashes, if he filled it with water that is not appropriate for mixing, as for example, the water of a Mikveh that are not potable, running spring water.

If one immersed a vessel for the hatat in water that is not fit for the mixing he must dry it; If in water that is fit for the mixing he need not dry it. The water used to mix in the ashes must be fresh water (see 19:17), water that has not been previously collected. If he takes a vessel that can be purified in a mikveh to use it for mixing in the hatat ash and he immerses it in a mikveh, he must make sure that it is completely dry before he puts in the water for the ritual. But if he immersed it in fresh water, he doesn't need to dry it because that water is fit in any case.

צריך לנגב – [he needs to wipe off] the vessel, for perhaps that which was appropriate would be mixed with that which is inappropriate. But if he immersed it in potable, running spring water waters, they are appropriate for mixing, one doesn’t need to dry it off.

But if [he intended] to collect in it water that was already mixed with the ashes, he must dry it in either case. If he immersed the vessel to gather water in which the ashes had already been mixed, then he needs to dry it no matter where he immersed it. This is because he needs to prevent water that has not been sanctified with the hatat (the water in which he immerses) with the water that has already been mixed.

ואם לאסוף לתוכו מים מקודשים – but if he immersed it in order to bring into water that he had already placed in it the ashes, even if he had immersed it in potable, running spring water, he needs to dry it, so that water that is not mixed becomes combined with water that is mixed, and it is taught in the Mishnah further on (Chapter 9, Mishnah 1 of Tractate Negaim) a flask of lustration that invalid water fell into it.