Musnad Ahmad 30:290

Yahya ibn Sa'id narrated to us, from Sufyan, who said: Salam ibn 'Abd al-Rahman narrated to me, from Abu Zur'ah, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) used to dislike the *shikal* (a white marking on three legs and one different) in horses.