Musa bin Ubayd (may Allah have mercy on him) said: After praying the dawn prayer we stayed in the Hijr until morning. When it had become fully light, we saw that Abdullah bin Umar (may Allah be pleased with them both) was also there. He was asking each man one by one: "Where did you pray the morning prayer?" He was asking everyone, until he asked me: "O Ibn Ubayd, where did you pray?" I said: "Here." He was pleased and said: "We do not know of any prayer of greater virtue than praying the morning prayer in congregation on Friday." They asked him: "O Abu Abd al-Rahman, did you use to wager on horse races in the time of the Prophet (peace and blessings be upon him)?" He said: "Yes; indeed, the Prophet (peace and blessings be upon him) wagered on a horse, and he (peace and blessings be upon him) won."
The Shaykh (may Allah have mercy on him) said: This, if it is authentic, means only that one of the two horsemen raced his companion with the stake coming from him alone, not from his companion. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السبق والرمي / 19775