Numbers 8:22-24

New Revised Standard Version, Anglicised

²² Thereafter the Levites went in to perform their service in the tent of meeting in attendance on Aaron and his sons. As the Lord had commanded Moses concerning the Levites, so they dealt with them.

²³ The Lord spoke to Moses, saying: ²⁴ This applies to the Levites: from twenty-five years old and upwards they shall begin to do duty in the service of the tent of meeting;