Numbers 3:40-41

40Then the Lord said to Moses, “Number every firstborn male of the Israelites from a month old and upward, and make a list of their names.

41You shall take the Levites for Me instead of and as a substitute for all the firstborn among the Israelites. I am the Lord; and you shall take the cattle of the Levites instead of and as a substitute for all the firstborn among the cattle of the Israelites.”