Numbers 3:38-39

38Those who were to camp before the tabernacle on the east, before the tent of meeting toward the sunrise, were Moses and Aaron and his sons, guarding the sanctuary itself, to protect the people of Israel. And any outsider who came near was to be put to death.

39All those listed among the Levites, whom Moses and Aaron listed at the commandment of the Lord, by clans, all the males from a month old and upward, were 22,000.