Numbers 8:20-22

20So Moses, Aaron and the entire community of Bnei-Yisrael did so with the Levites. All that Adonai had commanded Moses regarding the Levites, so Bnei-Yisrael did to them.

21The Levites also purified themselves from sin and washed their clothes. Aaron presented them as a wave offering before Adonai, and, he made atonement for them to purify them.

22After that, the Levites came to do their tasks before Aaron and his sons in the Tent of Meeting, just as Adonai had commanded Moses concerning the Levites.