Musnad Ahmad 773:5

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from 'Abdullah ibn Abi al-Safar, from al-Sha'bi, from 'Urwah ibn al-Ja'd, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Good is tied to the forelocks of horses until the Day of Resurrection: the reward and the spoils of war."