2 Chronicles 23:4

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, guarding the thresholds. It should be remembered that a certain part of the Levites retired from service every Sabbath, while an equal number resumed their ministry on that day. On this occasion they were apparently placed under the command of the centurions of the guards to insure proper military order, in case necessity should arise.