Musannaf Ibn Abi Shayba 28:1067

'Abd al-Salam ibn Harb narrated to us, from al-A'mash, from Ibrahim, who said: They used to dislike that one of them would say to his companion, "I will race you on the condition that if you beat me, then [I owe you something], and if I beat you, then it is mine; and if not, then it is upon you," for that is gambling.