Musnad Ahmad 30:3426

Yazid narrated to us, Sufyan ibn Husayn informed us, from al-Zuhri, from Sa'id ibn al-Musayyib, from Abu Hurayrah, from the Prophet, peace and blessings be upon him, who said: "Whoever enters a horse between two horses, while he does not feel secure that he will be outrun, then there is no harm in it. And whoever enters a horse between two horses, while he feels secure that he will not be outrun, then it is gambling."