הלכה אויף במדבר 19:21

That the waters of separation defile one who is clean, and cleanse the unclean from pollution by a dead body (Num. 19:19-22). With reference to these modes of uncleanness, most of the rules and regulations concerning each of them are explained in the Pentateuch.

The Commandment of the niddah waters which render the pure, impure and only purify someone impure from the impurity of a dead body: That we have been commanded about the laws of the niddah waters, meaning to say in the laws of the waters of sprinkling, which are “living” waters mixed with the ashes of the heifer, which we sprinkle on the impure. And the expression, “niddah,” is meaning to say, sprinkling, which is an expression of throwing, as in “and they threw (yadu) a stone at me” (Lamentations 3:53). And we were commanded with laws known from the Scripture: That we purify the impure, as it is stated (Numbers 19:19), “And the pure will sprinkle the impure, etc.”; and render the pure impure [with] a severe impurity, as it is written (Numbers 19:21), “and the one who sprinkles the niddah waters, etc.” And that which is stated about this commandment [to call it] the “statute” (chukat) of the heifer, is [because] it is a lofty wonder — I cannot master it.