Numbers 8:16-20

for they are exclusively given to Me from among the sons of Israel. I have taken them for Myself instead of the firstborn of every womb, the firstborn of all the sons of Israel.

For every firstborn among the sons of Israel is Mine, among the people and among the animals; on the day that I fatally struck all the firstborn in the land of Egypt, I sanctified them for Myself.

But I have taken the Levites instead of every firstborn among the sons of Israel.

And I have given the Levites as a gift to Aaron and to his sons from among the sons of Israel, to perform the service of the sons of Israel at the tent of meeting and to make atonement on behalf of the sons of Israel, so that there will be no affliction among the sons of Israel due to their approaching the sanctuary.”

So this is what Moses, Aaron, and all the congregation of the sons of Israel did to the Levites; according to everything that the Lord had commanded Moses regarding the Levites, so the sons of Israel did to them.