Musnad Abu Ya'la al-Mawsili 155:24

Muhammad ibn Yahya ibn Abi Samina narrated to us, Abbad ibn al-Awwam narrated to us, from Sufyan ibn Husayn, 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 hopes that it will win, there is no good in it. And whoever introduces a horse between two horses while he does not hope that it will win, there is no harm in it."