Numbers 3:46

44The LORD spoke to Moses, saying,

45“Take the Levites instead of all the firstborn among the children of Israel, and the livestock of the Levites instead of their livestock; and the Levites shall be mine. I am the LORD.

46For the redemption of the two hundred seventy-three of the firstborn of the children of Israel who exceed the number of the Levites,

47you shall take five shekels apiece for each one; according to the shekel of the sanctuary you shall take them (the shekel is twenty gerahs );

48and you shall give the money, with which their remainder is redeemed, to Aaron and to his sons.”