Numbers 3:38-39

38Moses, Aaron and Aaron's sons had to put up their tents in front of the tabernacle, on the east side. They were the only people who could go into the Holy Place in the Tent. Any other person who went near it had to die.

39So Moses and Aaron obeyed the Lord. They counted the Levites by their clans. There were 22,000 males who were one month old or older.