Numbers 19:9, 18

Then 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.

A clean person shall take hyssop and dip it in the water, sprinkle it on the tent, on all the vessels, on the persons who were there, or on the one who touched a bone, the slain, the dead, or a grave.