The First Epistle of Clement to the Corinthians

He commanded us to celebrate sacrifices and services, and that it should not be thoughtlessly or disorderly, but at fixed times and hours.

He has himself fixed by his supreme will the places and persons whom he desires for these celebrations, in order that all things may be done piously according to his good pleasure, and be acceptable to his will.

So then those who offer their oblations at the appointed seasons are acceptable and blessed, for

For to the High Priest his proper ministrations are allotted, and to the priests the proper place has been appointed, and on Levites their proper services have been imposed. The layman is bound by the ordinances for the laity.