Musnad Ahmad 32:684

Abu Kamil narrated to us, Sa'id ibn Zayd narrated to us, al-Zubayr ibn Khirrit narrated to us, Abu Labid Limazah ibn Zabbar narrated to us, who said: The horses were released during the time of al-Hajjaj, so we said: "If only we went to the racecourse." He said: So we went to it, then we said: "If only we turned to Anas ibn Malik and asked him: 'Did you used to bet on horse races during the time of the Messenger of Allah, peace and blessings of Allah be upon him?'" He said: So we went to him and asked him, and he said: "Yes, he indeed bet on a horse of his, which was called Subhah, and it outpaced the people." He was delighted by that, and it pleased him.