Musnad Ahmad 773:12

'Affan narrated to us, Shu'bah narrated to us, Husayn and 'Abdullah ibn Abi as-Safar informed me that they heard ash-Sha'bi, who heard 'Urwah ibn al-Ja'd, from the Prophet (peace and blessings of Allah be upon him), who said: "Good is tied to the forelocks of horses until the Day of Resurrection: the reward and the spoils of war."