Leviticus 8:5

And Moses said unto the congregation, This is the thing which the LORD commanded to be done.

Moses stood before the assembled congregation and declared that what was about to be done had been commanded by the Lord. His announcement introduced the consecration of Aaron to the office of high priest. That consecration was carried out in obedience to the Lord's command. The priesthood itself, and Aaron's sacred setting apart to it, originated from God Himself.