Musnad Ahmad 27:1

Abdullah ibn Ahmad narrated to us, my father narrated to me in his book, Hushaym ibn Bashir narrated to us, from Ubaydullah, and Abu Mu'awiyah, Ubaydullah informed us, from Nafi', from Ibn Umar (may Allah be pleased with them both): "That the Messenger of Allah (peace and blessings of Allah be upon him) assigned on the day of Khaybar two shares for the horse, and one share for the man." And Abu Mu'awiyah said: "He assigned for the man and his horse three shares: one share for him, and two shares for his horse."