Musnad Ahmad 1041:2

Yunus ibn Muhammad narrated to us, 'Abd al-Wahid ibn Ziyad narrated to us, Mujalid ibn Sa'id narrated to us, al-Sha'bi narrated to me, he said: I asked Ibn 'Umar, I said: "Does a camel and a cow suffice for seven [people]?" He said: "O Sha'bi, does it have seven souls?" He said: I said: "The companions of Muhammad claim that the Messenger of Allah (peace and blessings of Allah be upon him) ordained the camel and the cow for seven." He said: Then Ibn 'Umar said to a man: "Is it so, O so-and-so?" He said: "Yes." He said: "I was not aware of this."