Numbers 19:12, 19

He is to purify himself with the water  on the third day and the seventh day; then he will be clean.  But if he does not purify himself on the third and seventh days, he will not be clean.

“The one who is clean is to sprinkle the unclean person on the third day and the seventh day. After he purifies the unclean person on the seventh day, the one being purified must wash his clothes and bathe in water, and he will be clean by evening.