《礼记·郊特牲》
天子到诸侯国，诸侯供膳只用一头小牛。而诸侯朝见天子，天子设宴则用牛、羊、猪三牲。
天子适诸侯，诸侯膳用犊；诸侯适天子，天子赐之礼大牢；贵诚之义也。
When the son of Heaven went to a state, he was entertained with an ox. When the princes of a state went to court, they were feasted with animals of the three classes—oxen, sheep and pigs.