Musannaf Ibn Abi Shayba 28:958

Waki' narrated to us, he said: Sufyan narrated to us, from Yunus, from 'Amr ibn Sa'id, from Abu Zur'ah ibn 'Amr ibn Jarir, from Jarir, he said: I saw the Prophet, may Allah bless him and grant him peace, twisting the forelock of his horse with his finger, and he was saying: "Good is tied to the forelocks of horses until the Day of Resurrection: the reward and the spoils of war."