Numbers 8:17-19

17For all the firstborn among the children of Israel are mine, both of man and beast; on the day that I smote every firstborn in the land of Egypt, I hallowed them to myself.

18And I have taken the Levites instead of all the firstborn among the children of Israel.

19And I have given the Levites as a gift to Aaron and to his sons, from among the children of Israel, to perform the service of the children of Israel in the tent of meeting, and to make atonement for the children of Israel; that there be no plague among the children of Israel, when the children of Israel draw near to the sanctuary.