《礼记·郊特牲》
郊祀祭天时用一头牛，而祭祀土神和谷神时要用牛、羊、猪三牲。
郊特牲，而社稷大牢。
An ox was used for the sacrifice to Heaven, and animals of the three classes—oxen, sheep and pigs—for the sacrifices to the Earth and the Spirits of the land and grain.