Numbers 8:17-19

17For all the firstborn among the children of Israel are Mine, both man and beast; on the day that I struck all the firstborn in the land of Egypt I sanctified them to Myself.

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

19And I have given the Levites as a gift to Aaron and his sons from among the children of Israel, to do the work for the children of Israel in the tabernacle 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 come near the sanctuary.”