Numbers 8:18

15 And after that shall the Levites go in to perform the service of the tabernacle of the congregation: and thou shalt cleanse them, and offer them [for] an offering.

16 For they [are] wholly given to me from among the children of Israel; instead of such as open every womb, [even instead of] the first-born of all the children of Israel, have I taken them to me.

17 For all the first born of the children of Israel [are] mine, [both] man and beast: on the day that I smote every first-born in the land of Egypt I sanctified them for myself.

18 And I have taken the Levites for all the first-born of the children of Israel.

19 And I have given the Levites [as] a gift to Aaron and to his sons from among the children of Israel, to do the service of the children of Israel in the tabernacle of the congregation, and to make an atonement for the children of Israel: that there may be no plague among the children of Israel, when the children of Israel come nigh to the sanctuary.

20 And Moses, and Aaron, and all the congregation of the children of Israel, did to the Levites according to all that the LORD commanded Moses concerning the Levites, so did the children of Israel to them.

21 And the Levites were purified, and they washed their clothes; and Aaron offered them [as] an offering before the LORD; and Aaron made an atonement for them to cleanse them.