2 Chronicles 23:2-6

2And they went about in Judah, and gathered the Levites out of all the cities of Judah, and the heads of fathers’ houses of Israel, and they came to Jerusalem.

3And 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 spoken concerning the sons of David.

4This is the thing that ye shall do: a third part of you, that come in on the sabbath, of the priests and of the Levites, shall be porters of the doors;

5and 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.

6But let none come into the house of the LORD, save the priests, and they that minister of the Levites; they shall come in, for they are holy: but all the people shall keep the watch of the LORD.