Numbers 8:11-13

And Aaron shall offer the Levites before the LORD for an offering of the children of Israel, that they may execute the service of the LORD. Aaron shall offer the Levites ... for an offering, х tªnuwpaah (H8573)] - a wave offering; and it has been thought probable that the high priest, in bringing the Levites one by one to the altar, directed them to make some simple movements of their persons, analogous to what was done at the presentation of the wave offerings before the Lord. Thus were they first devoted as an offering to God, and by Him surrendered to the priests to be employed in His service. The consecration ceremonial was repeated in the case of every Levite who was taken, as at a later period was done, to assist the priests in the tabernacle and temple. (See the note at 2 Chronicles 29:24).