Leviticus 8:14

(14) And he brought the bullock for the sin offering—Though duly consecrated, Aaron and his sons had first to be purged of their sins before they could commence their priestly functions in the sanctuary. Hence, Moses, as the mediator of the covenant delegated by God to perform the act of consecration, also performed the sacrificial rites, whilst the installed priests stood as penitent sinners by the side of the sin offering which was now offered for the first time. For the laying on of the hands by the offerer on the victim, see Leviticus 1:4.