Numbers 3:44-47

44Then the 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. The Levites shall be Mine: I am the Lord.

46And for the redemption of the two hundred and seventy-three of the firstborn of the children of Israel, who are more than the number of the Levites,

47you shall take five shekels for each one individually; you shall take them in the currency of the shekel of the sanctuary, the shekel of twenty gerahs.