Numbers 3:12, 40-41

“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

Then the LORD said to Moses, “Number all the firstborn males of the Israelites from a month old and upward, and take the number of their names. And take the Levites for me – I am the LORD – instead of all the firstborn males among the Israelites, and the livestock of the Levites instead of all the firstborn of the livestock of the Israelites.”