Numbers 19:9-10

9Now a man who is [ceremonially] clean shall collect the ashes of the heifer and deposit them outside the camp in a clean place, and the congregation of the Israelites shall keep it for water to remove impurity; it is [to be used for] purification from sin.

10The one who gathers the ashes of the heifer shall wash his clothes, and be unclean until evening. This shall be a perpetual statute to the Israelites and to the stranger who lives as a resident alien among them.