Musnad Ahmad 30:1572

Ishaq narrated to us, Ibn Lahi'ah narrated to us, from Abu al-Aswad, who said: I asked Sulayman ibn Yasar about the *sabq* (wager/prize for racing), and he said: Abu Salih narrated to me, he said: I heard Abu Hurayrah say: I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "There is no *sabq* (wager) except in a hoof, or a hoof (i.e., camel or horse)."