Exodus 27:21; Exodus 29:9

21 vIn the tent of meeting, woutside the veil that is before the testimony, Aaron and his sons shall tend it from evening to morning before the Lord. It shall be a statute forever to be observed throughout their generations by the people of Israel.

and you shall gird Aaron and his sons with jsashes and bind caps on them. And kthe priesthood shall be theirs by a statute forever. Thus you shall lordain Aaron and his sons.