Numbers 3:45-48

Take the Levites instead of all the first-born among the children of Israel, and the cattle of the Levites instead of their cattle; and the Levites shall be mine: I am Jehovah. And for the redemption of the two hundred and threescore and thirteen of the first-born of the children of Israel, that are over and above the number of the Levites, thou shalt take five shekels apiece by the poll; after the shekel of the sanctuary shalt thou take them (the shekel is twenty gerahs): and thou shalt give the money, wherewith the odd number of them is redeemed, unto Aaron and to his sons.