Leviticus 9:8

"So Aaron drew near the altar, and slew the calf of the sin-offering, which was for himself. And the sons of Aaron delivered unto him the blood; and he dipped his finger in the blood, and put it upon the horns of the altar, and poured out the blood at the base of the altar: but the fat, and the kidneys, and the caul of the liver of the sin-offering, he burnt upon the altar; as Jehovah commanded Moses. And the flesh and the skin he burnt with fire without the camp."

Aaron here followed the directions laid down in Leviticus 1-7 regarding sin-offerings, observing the restrictions that priests could not eat the flesh of their own sin-offerings, hence the burning of the flesh and the skin without (outside) the camp. The skin also, in usual instances, would have belonged to the officiating priest.

Note that all four of Aaron's sons were assistants and witnesses of this solemn ordination of the high priest. Christ was also assisted by the four witnesses of the Christian Gospel - Matthew, Mark, Luke and John - but even as was to be the case here, the four witnesses fall into groups of two each - Matthew and John were apostles; Luke and Mark were not apostles.