Musannaf Ibn Abi Shayba 28:957

Ghundar narrated to us, from Shu'bah, from Ibn Abi al-Safar, from 'Urwah al-Bariqi, who said: I heard the Messenger of Allah (may Allah bless him and grant him peace) say: "Good is tied to the forelocks of horses until the Day of Resurrection: the reward and the spoils of war."