מדרש על ויקרא 21:24

12) (Vayikra 21:24) "And Moses spoke to Aaron and to his sons and to all the children of Israel": He exhorted Aaron ( in respect to blemishes) by way of the sons, and the sons by way of Aaron, and the sons by each other (i.e., by mutual surveillance), and the (beth-din of the) Israelites over them (the Cohanim).