Parah, Chapter 5, Mishnah 2. This chapter deals with the vessels used in preparing the chatas waters, the waters into which the ashes of the Parah Adumah are placed. Our mishnah addresses a detail that arises after a vessel has been immersed in the mikveh: the water still clinging to its inside.
The Mishnah teaches: "Hamatbil kli l'chatas bemayim she'einan re'uyim l'kadesh, tzarich l'nageiv" - whoever dips a chatas vessel into water unsuitable for kiddush, the act of sanctifying, is obligated to wipe that vessel out. "Bemayim shehem re'uyim l'kadesh, eino tzarich l'nageiv" - where the immersion took place in water suitable for kiddush, no wiping is required of him. "Im l'hosif l'socho mayim mekudashim, bein kach u'vein kach tzarich l'nageiv" - but if the intention is to pour ash-sanctified water into that vessel, wiping is required in either situation.
Which water was he immersed in?
The case is a vessel that has been prepared for the chatas, to be filled with water and sanctified with the ashes of the Parah Adumah. Before use it is immersed, and the question is what kind of water it was immersed in.
If it was immersed in water that is not fit for the sanctification of the chatas waters, that is, ordinary mikveh water rather than water drawn from a spring, the residue left inside the vessel is a problem, and he must dry the vessel out before using it.
If, however, the immersion took place in water that is itself fit for sanctification, spring water, there is nothing objectionable clinging to the walls of the vessel, and he need not dry it at all.
When sanctified water is being added
The final clause is more demanding. If this vessel is being prepared to receive water that has already been sanctified with the ashes, poured into it from elsewhere, then whichever water it was immersed in, fit or unfit, the vessel must be dried first.
The Mishnah's message is characteristic of the laws of the chatas: even a few drops left in the bottom of a vessel are weighed carefully, because the purity and integrity of the chatas waters depend on details that would pass unnoticed anywhere else.