Musnad Ahmad 770:45

Hushaym narrated to us, Yunus informed us, from ‘Amr ibn Sa‘id, from Abu Zur‘ah ibn ‘Amr, from Jarir ibn ‘Abd Allah, who said: I saw the Messenger of Allah, may Allah bless him and grant him peace, twisting the forelock of a horse with his two fingers while he was saying: "Good is tied to the forelocks of horses—the reward and the spoils—until the Day of Resurrection."