Numbers 19:19-20

19The man who is clean is to sprinkle those who are unclean on the third and seventh days, and on the seventh day he is to purify them. Those who are being cleansed must wash their clothes and bathe with water, and that evening they will be clean.

20But if those who are unclean do not purify themselves, they must be cut off from the community, because they have defiled the sanctuary of the Lord. The water of cleansing has not been sprinkled on them, and they are unclean.