Musnad Ahmad 773:6

Yahya ibn Sa'id narrated to us, from Zakariyya, and Waki' said: Zakariyya narrated to us, from 'Amir, from 'Urwah—Yahya said: Ibn Abi al-Ja'd al-Bariqi—from the Prophet, peace and blessings of Allah be upon him. And Waki' said in his narration: I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "Good is tied to the forelocks of horses until the Day of Resurrection: the reward and the spoils of war."