Numbers 19:12, 19

They must purify themselves on the third and seventh days with the water of purification; then they will be purified. But if they do not do this on the third and seventh days, they will continue to be unclean even after the seventh day.

On the third and seventh days the person who is ceremonially clean must sprinkle the water on those who are defiled. Then on the seventh day the people being cleansed must wash their clothes and bathe themselves, and that evening they will be cleansed of their defilement.