2 Chronicles 35:15

And the singers, the sons of Asaph, were in their place, in the station appointed to them, where they chanted their psalms, according to the commandment of David, and Asaph, and Heman, and Jeduthun, the king's seer. And the porters waited at every gate, both the singers and the porters being on duty, of course, on every day of the feast; they might not depart from their service; for their brethren, the Levites, prepared for them, so they all could take part in the great sacrificial feast.