Exodus 29:9

29:9 and wrap the sashes around Aaron and his sons and put headbands on them, and so the ministry of priesthood will belong to them by a perpetual ordinance. Thus you are to consecrate Aaron and his sons.

Exodus 29:28

29:28 It is to belong to Aaron and to his sons from the Israelites, by a perpetual ordinance, for it is a contribution. It is to be a contribution from the Israelites from their peace offerings, their contribution to the Lord.

Exodus 30:21

30:21 they must wash their hands and their feet so that they do not die. And this will be a perpetual ordinance for them and for their descendants throughout their generations.”