Exodus 29:9-11

And thou shalt gird them with girdles, Aaron and his sons, and put the bonnets on them: and the priest's office shall be their's for a perpetual statute: and thou shalt consecrate Aaron and his sons. And thou shalt cause a bullock to be brought before the tabernacle of the congregation: and Aaron and his sons shall put their hands upon the head of the bullock. And thou shalt kill the bullock before the LORD, by the door of the tabernacle of the congregation.