Nehemiah 12:45

And both the singers and the porters kept the ward of their God, and the ward of the purification, according to the commandment of David, and of Solomon his son.

The singers and the porters kept ... the ward of the purification - i:e., took care that no unclean person was allowed to enter within the precincts of the sacred building. This was the official duty of the porters (2 Chronicles 23:19), with whom, owing to the pressure of circumstances, it was deemed expedient that the singers should be associated as assistants.