Ibn Ezra on Numbers 31:23

אך במי נדה יתחטא. היה נראה לנו כי הוא מי אפר הפרה כמו מי נדה לא זורק עליו וחז"ל אמרו כי טעמו בשיעור המים שתרחץ בהם האשה הנדה ודעתם רחבה מדעתינו: NEVERTHELESS IT SHALL BE PURIFIED WITH THE WATER OF SPRINKLING.³⁵Hebrew, me niddah. It would have appeared to us that the reference is to the mixture of the water and tahe ashes of the cow,³⁶The red heifer. as in because the water of sprinkling³⁷Hebrew, me-niddah. was not dashed against him (Num. 19:13). However, our wise men of blessed memory say that its meaning is, in the amount of water in which a menstruant bathes.³⁸The utensils are to be immersed in a pool of water which is fit for a menstruant to purify herself. See Avodah Zara 75b. Their mind is much greater than our minds.