Musnad Ahmad 30:3030

Waki’ narrated to us, he said: Sufyan narrated to us, from Salam ibn ‘Abd al-Rahman al-Nakha’i, from Abu Zur’ah ibn ‘Amr ibn Jarir, from Abu Hurairah: "That the Prophet, peace and blessings of Allah be upon him, used to dislike the *shikal* (a white marking on three legs and one leg of a different color) in horses."