Numbers 8:13-19

13Have the Levites stand before Aaron and his sons and present them as a wave offering to Adonai.

14In this way you are to set apart the Levites from Bnei-Yisrael to be Mine.

15After you have purified them and presented them as a wave offering, the Levites will come to do their work at the Tent of Meeting.

16For they are the ones from among Bnei-Yisrael given to Me in place of all the first from the wombs of Bnei-Yisrael. I have taken them for Myself.

17“For every firstborn among Bnei-Yisrael is Mine, whether human or animal. On the day I struck down the firstborn of the land of Egypt, I sanctified them for Myself.

18So I am taking the Levites in place of the firstborn of Bnei-Yisrael,

19and I am giving the Levites as a gift to Aaron and his sons from among Bnei-Yisrael, to do the work on behalf of Bnei-Yisrael in the Tent of Meeting, and to make atonement for them—so that there would be no plague among them for coming too close to the Sanctuary.”