Exodus 40:15 You shall anoint them, as you anointed their father, that they may minister to me in the priest's office. Their anointing shall be to them for an everlasting priesthood throughout their generations." (See NAS)
Numbers 8:11 and Aaron shall offer the Levites before Yahweh for a wave offering, on the behalf of the children of Israel, that it may be theirs to do the service of Yahweh. (See NAS)