Leviticus 9:7

קרב 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). את חטאתך THY SIN OFFERING — the young calf (v. 2). ואת עלתך AND THY BURNT OFFERING — the ram (v. 2). קרבן 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).

וַיֹּ֨אמֶר מֹשֶׁ֜ה אֶֽל־אַהֲרֹ֗ן קְרַ֤ב אֶל־הַמִּזְבֵּ֙חַ֙ וַעֲשֵׂ֞ה אֶת־חַטָּֽאתְךָ֙ וְאֶת־עֹ֣לָתֶ֔ךָ וְכַפֵּ֥ר בַּֽעַדְךָ֖ וּבְעַ֣ד הָעָ֑ם וַעֲשֵׂ֞ה אֶת־קׇרְבַּ֤ן הָעָם֙ וְכַפֵּ֣ר בַּֽעֲדָ֔ם כַּאֲשֶׁ֖ר צִוָּ֥ה יְהֹוָֽה׃

And Moshe said to Aharon, Go to the altar, and offer thy sin offering, and thy burnt offering, and make atonement for thyself, and for the people: and offer the offering of the people, and make atonement for them; as the Lord commanded.