Musnad Ahmad 529:1

'Abd al-Razzaq narrated to us, Sufyan informed us, from Thawr ibn Yazid, from Nasr, from a man, from 'Utbah ibn 'Abd al-Sulami, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) forbade the plucking of horses' tails, their manes, and their forelocks." And he said: "Their tails are their fly-whisks, their manes are their warmth, and their forelocks have goodness tied to them until the Day of Resurrection."