Numbers 19:4, 19

Eleazar will take some of its blood on his finger and sprinkle it seven times toward the front of the Tabernacle.

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.