1 Chronicles 26:18

The singers, the sons of Asaph, were also at their stations according to the commandment of David, Asaph, Heman, and Jeduthun the king’s seer; and the gatekeepers at each gate did not have to turn aside from their service, because the Levites their brothers prepared for them.

“Yet they shall be ministers in My sanctuary, having oversight at the gates of the house and ministering in the house; they shall slaughter the burnt offering and the sacrifice for the people, and they shall stand before them to minister to them.