Musnad Ahmad 33:478

Yunus ibn Muhammad narrated to us, 'Abd al-Wahid narrated to us, Mujalid ibn Sa'id narrated to us, al-Sha'bi narrated to me, Jabir ibn 'Abd Allah narrated to me: "The Messenger of Allah (may Allah bless him and grant him peace) ordained the camel and the cow to be [sacrificed] on behalf of seven."