Numbers 3:12, 40-41

“Look, I have chosen the Levites from among the Israelites to serve as substitutes for all the firstborn sons of the people of Israel. The Levites belong to me

Then the LORD said to Moses, “Now count all the firstborn sons in Israel who are one month old or older, and make a list of their names. The Levites must be reserved for me as substitutes for the firstborn sons of Israel; I am the LORD. And the Levites’ livestock must be reserved for me as substitutes for the firstborn livestock of the whole nation of Israel.”