Numbers 8:12-16

Now the Levites shall lay their hands on the heads of the bulls; then you are to offer the one as a sin offering and the other as a burnt offering to the Lord, to make atonement for the Levites.

And you shall have the Levites stand before Aaron and his sons so as to present them as a wave offering to the Lord.

“So you shall single out the Levites from among the sons of Israel, and the Levites shall be Mine.

Then after that the Levites may go in to serve the tent of meeting. But you shall cleanse them and present them as a wave offering;

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.