Musannaf Ibn Abi Shayba 28:45

Waki’ narrated to us, he said: Thawr al-Shami narrated to us, from al-Wadin ibn ‘Ata’, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Do not cut the tails of horses, for they are their fly-swatters. And do not clip their manes, for they are their warmth."