Musnad Ahmad 1034:212

Yahya ibn Adam narrated to us, Isra'il narrated to us, al-Hakam ibn 'Utaybah narrated to us, from al-Mughirah ibn Hudhaf, from Hudhayfah, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, shared [the sacrifice] among the Muslims in his Hajj, for every seven [people] one cow."