Numbers 8
The Lord instructed Moses regarding making the Levites "ceremonially clean" that they would be purified and may be "ready to do the work of the Lord". The Lord said the Levites were to be "set apart from the other Israelites" and they "will be mine" and "are to be given wholly to me.. in place of the firstborn sons of Israel..and I have given the Levites as gifts to Aaron to do the work at the Tent of Meeting to make atonement for them."