Numbers 19:18-20 New Revised Standard Version Catholic Edition (NRSVCE)
18 then a clean person shall take hyssop, dip it in the water, and sprinkle it on the tent, on all the furnishings, on the persons who were there, and on whoever touched the bone, the slain, the corpse, or the grave. 19 The clean person shall sprinkle the unclean ones on the third day and on the seventh day, thus purifying them on the seventh day. Then they shall wash their clothes and bathe themselves in water, and at evening they shall be clean. 20 Any who are unclean but do not purify themselves, those persons shall be cut off from the assembly, for they have defiled the sanctuary of the Lord. Since the water for cleansing has not been dashed on them, they are unclean.