Halakhah for Numbers 19:21

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.