Musnad Ahmad 529:3

Abdullah ibn al-Harith narrated to us, Thawr ibn Yazid narrated to me, from Nasr, from a man of Banu Sulaym, from Utbah ibn Abd al-Sulami: That the Prophet, peace and blessings of Allah be upon him, forbade the clipping of the manes of horses, the plucking of their tails, and the clipping of their forelocks. He said: "As for their tails, they are their fly-swatters; as for their manes, they are their warmth; and as for their forelocks, goodness is tied to them."