Numbers 8:21

Now when the children of Israel had done so to them, and the Levites were purified, and had washed their clothes, and Aaron had solemnly given them up in the presence of the Lord, and had made atonement for them to purify them;

So the Levites purified themselves and washed their garments; and Aaron presented them as a gift before the Lord, and Aaron made atonement for them to purify them.