Numbers 19:9, 18

Now 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.

A clean person shall take hyssop and dip it in the water and sprinkle it on the tent and on all the furnishings and on the people who were there, and on the one who touched the bone or the one who was killed or the one who died [naturally] or the grave.