And the Lord spoke to Moses and said: Take the Levites from among the children of Israel and cleanse them. You shall absolve them by sprinkling water upon them, shave all their hair off completely and wash their clothes. Then take one young bull and mix it with flour and oil for a food offering. Thou shalt take another young bull for a sin offering. Then bring the Levites before the tabernacle as well as the whole of the Israeli community. Thou shalt lay your hands on the Levites. And Aaron shall offer the Levites a swing offering before the Lord. The Levites shall lay their hands on the head of the young bull. This is how thou shalt separate the Levites from the children of Israel. Then the Levites shall go and perform their duties at the tabernacle. Because they are given to me as a gift from among the children of Israel.
I have taken them instead of all the firstborn of Israel. I sanctified them to me, even in the land of Egypt, so that they should run the liturgy for the children of Israel, and that there would be no plague among them when they should or wanted to come near the sanctuary. Moses and Aaron did what the Lord commanded.

That is, said the Lord, what applies to the Levites: They shall begin service from twenty five years old in order to perform their duties. From the age of fifty, they shall be free from the sanctuary, and shall only help their brothers with the service at the tabernacle.