Numbers 8:16-19

for they are wholly given to Me from among the Israelites. I have taken them for Myself instead of all who are born first, the firstborn of all the Israelites. For all the firstborn among the Israelites are Mine, among the men and among the animals; on the day that I struck down every firstborn in the land of Egypt, I sanctified and set the Israelites apart for Myself. And I have taken the Levites instead of every firstborn among the Israelites. I have given the Levites as a gift to Aaron and to his sons from among the Israelites, to perform the service of the Israelites at the Tent of Meeting (tabernacle) and to make atonement for them, so that there will be no plague among the Israelites if they should approach the sanctuary.”