Musnad Ahmad 27:731

Yahya narrated to us, from Ubayd Allah, Nafi’ informed me, from Ibn Umar, who said: "The Prophet of Allah, peace and blessings of Allah be upon him, sent us in a military expedition. Our shares reached twelve camels, and the Messenger of Allah, peace and blessings of Allah be upon him, gave us an extra camel each as a reward (nafal)."