Numbers 3:48

And thou shalt give the money, wherewith the odd number of them is to be redeemed, unto Aaron and to his sons.

Cross-reference

Numbers 3:39 provides the total Levite count (22,000) which determines the excess firstborn requiring redemption here.

Numbers 3:51 records the fulfillment of giving the redemption money to Aaron, showing obedience to this command.