Numbers 8:16-18

16For they are wholly given unto me from among the children of Israel; instead of every one that breaketh open the womb, instead of every firstborn among the children of Israel, have I taken them unto me.

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.