Numbers 19:18-19

18Then a clean person will take some hyssop, dip it into the water, and, sprinkle it on the tent, all of the furnishings, and the people who were there, as well as the one touching the bone, the one killed, the corpse or the grave.

19The clean person will sprinkle the unclean one on the third and seventh days. He is to purify himself on the seventh day, and on the seventh day he is to wash his clothes and bathe himself in water, and at evening he will be clean.