Numbers 3:12, 40-41

“See, I have taken the Levites from the Israelites in place of every firstborn Israelite from the womb.  The Levites belong to me

The LORD told Moses, “Register every firstborn male of the Israelites  one month old or more, and list their names. You are to take the Levites for me — I am the LORD  — in place of every firstborn among the Israelites, and the Levites’ cattle in place of every firstborn among the Israelites’ cattle.”