Musnad Ahmad 30:3008

Yahya narrated to us, from Ibn Abi Dhi'b, from Nafi' ibn Abi Nafi', who said: I heard Abu Hurairah say: The Messenger of Allah (peace and blessings of Allah be upon him) said: "There is no competition (for a prize) except in a hoof, or an arrowhead, or a hoof." And Waki' and Yazid narrated to us, from Ibn Abi Dhi'b, from Nafi' ibn Abi Nafi', the freed slave of Abu Ahmad, from Abu Hurairah, from the Prophet (peace and blessings of Allah be upon him), the like of it.