Numbers 8:12-14

“Have the Levites place their hands on the heads of the bulls, selecting one for the Absolution-Offering and another for the Whole-Burnt-Offering to GOD to make atonement for the Levites. Then have the Levites stand in front of Aaron and his sons and present them as a Wave-Offering to GOD. This is the procedure for setting apart the Levites from the rest of the People of Israel; the Levites are exclusively for my use.