Midras Imamat 8:15: Sifra

14) (Vayikra 8:15) ("And he slaughtered, and Moses took the blood and he put it on the horns of the altar roundabout with his finger. And he purified the altar, and the (remaining) blood he poured out at the base of the altar, and he sanctified it to make atonement upon it.") "And he slaughtered, and Moses took the blood": All the seven days of miluim Moses served in the high-priesthood. He slaughtered, he sprinkled, he purified, he poured, he atoned. This is the intent of "And he slaughtered, and Moses, took, etc." To what may this be compared? To (the instance of) a princess who was wed as a minor, it being stipulated with her mother that she (the mother) officiate until her daughter had learned (the protocols of majesty). Here, too, Aaron, at first was a Levite, viz.: (Shemoth 4:14): "Is not Aaron your brother the Levite?" When he was chosen to be high-priest, the Holy One Blessed be He said to Moses: You serve Me until Aaron learns (the service). Moses slaughtered and Aaron observed him; he sprinkled (the blood) and Aaron observed him; he sprinkled (the anointing oil on the altar) and Aaron observed him; he purified (the altar) and Aaron observed him; he poured out (the blood at the base of the altar) and Aaron observed him; he atoned and Aaron observed him.

15) "And he slaughtered and Moses took the blood … and he sanctified it (the altar) to make atonement upon it.": I do not know what this atonement is. Moses said (When the L–rd of the world ordained donations for the sanctuary, the Israelites prodded each other and they brought offerings unwillingly.) Moses said: Let this (blood) serve as atonement, that there not be theft in the sanctuary. And thus is it written (Isaiah 61:8): "For I am the L–rd. I love justice and hate theft (even) for (the bringing of) a burnt-offering."