Numbers 3:18

Numbers 3 records the generations of Aaron and Moses and the numbering of the tribe of Levi by its clans - Gershon, Kohath, and Merari - assigning each family its duties in carrying and guarding the tabernacle under Aaron's sons. The Lord declares that the Levites are given to Aaron and his sons in place of the firstborn of Israel, who had been consecrated to Him. Because the Levites were slightly fewer than the firstborn, the excess firstborn were redeemed with five shekels each, and Moses gave the redemption money to Aaron and his sons.