Numbers 19:12, 19

They must purify themselves with the water on the third day and on the seventh day; then they will be clean. But if they do not purify themselves on the third and seventh days, they will not be clean.

The 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.