Numbers 19:9-10

9Then a man who is clean shall gather up the ashes of the heifer, and store them outside the camp in a clean place; and they shall be kept for the congregation of the children of Israel for the water of purification; it is for purifying from sin.

10And the one who gathers the ashes of the heifer shall wash his clothes, and be unclean until evening. It shall be a statute forever to the children of Israel and to the stranger who dwells among them.