2 Chronicles 23:2-6

And they went about in Judah, and gathered the Levites out of all the cities of Judah, and the chief of the fathers of Israel, and they came to Jerusalem.

And all the congregation made a covenant with the king in the house of God. And he said unto them, Behold, the king’s son shall reign, as the LORD hath said of the sons of David.

This is the thing that ye shall do; A third part of you entering on the sabbath, of the priests and of the Levites, shall be porters of the doors;

And a third part shall be at the king’s house; and a third part at the gate of the foundation: and all the people shall be in the courts of the house of the LORD.

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.