Musannaf Ibn Abi Shayba 28:1024

Yazid ibn Harun narrated to us, he said: Sufyan ibn Husayn informed us, from al-Zuhri, from Sa'id ibn al-Musayyib, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Whoever introduces a horse between two horses while he is secure that he will be outrun, then it is gambling. And whoever introduces a horse between two horses while he is not secure that he will be outrun, then it is not gambling."