Numbers 19:9-10

Now a man who is clean shall gather up the ashes of the heifer and put them outside the camp in a clean place, and the congregation of the sons of Israel shall keep them for water to remove impurity; it is purification from sin. And the one who gathers the ashes of the heifer shall wash his clothes and will be unclean until evening; and it shall be a permanent statute for the sons of Israel and for the stranger who resides among them.