Rashi on Leviticus 9:7/רש"י על ויקרא ט׳:ז׳

1קרב אל המזבח [AND MOSES SAID TO AARON] GO TO THE ALTAR, for Aaron was diffident and feared to go there. Moses therefore said to him “Wherefore art thou diffident? For this purpose hast thou been selected!” (cf. Sifra, Shemini, Mechilta d'Miluim 2 8).

2את חטאתך THY SIN OFFERING — the young calf (v. 2).

3ואת עלתך AND THY BURNT OFFERING — the ram (v. 2).

4קרבן העם THE OFFERING OF THE PEOPLE — the kid of the goats and the calf and the lamb (v. 3). Wherever the term עגל, “calf” is mentioned without further definition it denotes one in its first year, and it is from this passage that you may derive this rule (cf. Sifra on 4:2; Rosh Hashanah 10a).