Numbers 19:18-20

19:18 Then a ceremonially clean person must take hyssop, dip it in the water, and sprinkle it on the tent, on all its furnishings, and on the people who were there, or on the one who touched a bone, or one killed, or one who died, or a grave. 19:19 And the clean person must sprinkle the unclean on the third day and on the seventh day; and on the seventh day he must purify him, ¹  and then he must wash his clothes, and bathe in water, and he will be clean in the evening. 19:20 But the man who is unclean and does not purify himself, that person must be cut off from among the community, because he has polluted the sanctuary of the Lord; the water of purification was not sprinkled on him, so he is unclean.

¹ tn The construction uses a simple Piel of חָטָא (khata’, “to purify”) with a pronominal suffix – “he shall purify him.” Some commentators take this to mean that after he sprinkles the unclean then he must purify himself. But that would not be the most natural way to read this form.