TSK · Numbers 8:21

“Look, I myself have taken the Levites from among the Israelites instead of every firstborn who opens the womb among the Israelites. So the Levites belong to me,

and Aaron is to offer the Levites before the LORD as a wave offering from the Israelites, that they may do the work of the LORD.

He must purify himself with water on the third day and on the seventh day, and so will be clean. But if he does not purify himself on the third day and the seventh day, then he will not be clean.