Numbers 8:23-26

Now the LORD spoke to Moses, saying, “This is what applies to the Levites: from twenty-five years old and upward they shall enter to perform service in the work of the tent of meeting. But at the age of fifty years they shall retire from service in the work and not work any more. They may, however, assist their brothers in the tent of meeting, to keep an obligation, but they themselves shall do no work. Thus you shall deal with the Levites concerning their obligations.”