Numbers 8:13-23

13 Have the Levites stand in front of Aaron and his sons and then present them as a wave offering to the LORD.
14 In this way you are to set the Levites apart from the other Israelites, and the Levites will be mine.
15 “After you have purified the Levites and presented them as a wave offering, they are to come to do their work at the tent of meeting.
16 They are the Israelites who are to be given wholly to me. I have taken them as my own in place of the firstborn, the first male offspring from every Israelite woman.
17 Every firstborn male in Israel, whether human or animal, is mine. When I struck down all the firstborn in Egypt, I set them apart for myself.
18 And I have taken the Levites in place of all the firstborn sons in Israel.
19 From among all the Israelites, I have given the Levites as gifts to Aaron and his sons to do the work at the tent of meeting on behalf of the Israelites and to make atonement for them so that no plague will strike the Israelites when they go near the sanctuary.”
20 Moses, Aaron and the whole Israelite community did with the Levites just as the LORD commanded Moses.
21 The Levites purified themselves and washed their clothes. Then Aaron presented them as a wave offering before the LORD and made atonement for them to purify them.
22 After that, the Levites came to do their work at the tent of meeting under the supervision of Aaron and his sons. They did with the Levites just as the LORD commanded Moses.
23 The LORD said to Moses,