2 Chronicles 23:6
But let none come into the house of the Lord, save the priests, and they that minister of the Levites; they shall go in, for they are holy: but all the people shall keep the watch of the Lord.
Cross-reference
1 Chronicles 23:28–32
Related theme
1 Chronicles 23:28-32 outlines Levitical duties including guarding, providing background for the restriction here.