Numbers 8:18

14 In this way you shall separate the Levites from the rest of the Israelites, and the Levites will belong to Me.

15 After you have cleansed them and presented them as a wave offering, they may come to serve at the Tent of Meeting.

16 For the Levites have been wholly given to Me from among the sons of Israel. I have taken them for Myself in place of all who come first from the womb, the firstborn of all the sons of Israel.

17 For every firstborn male in Israel is Mine, both man and beast. I set them apart for Myself on the day I struck down all the firstborn in the land of Egypt.

18 But I have taken the Levites in place of all the firstborn among the sons of Israel.

19 And I have given the Levites as a gift to Aaron and his sons from among the Israelites, to perform the service for the Israelites at the Tent of Meeting and to make atonement on their behalf, so that no plague will come against the Israelites when they approach the sanctuary.”

20 So Moses, Aaron, and the whole congregation of Israel did with the Levites everything that the LORD had commanded Moses they should do.

21 The Levites purified themselves and washed their clothes, and Aaron presented them as a wave offering before the LORD. Aaron also made atonement for them to cleanse them.

22 After that, the Levites came to perform their service at the Tent of Meeting in the presence of Aaron and his sons. Thus they did with the Levites just as the LORD had commanded Moses.

Read all of Numbers 8