Numbers 19:9, 18

And a man that is clean shall gather up the ashes of the heifer, and lay them up without the camp in a clean place; and it shall be kept for the congregation of the children of Israel for a water for impurity: it is a sin-offering.

and a clean person shall take hyssop, and dip it in the water, and sprinkle it upon the tent, and upon all the vessels, and upon the persons that were there, and upon him that touched the bone, or the slain, or the dead, or the grave