Abu Dawud Book 27, Hadith Number 4067.
Chapter : Turbans.
Narated By Ali ibn Rukanah : Ali quoting his father said: Rukanah wrestled with the Prophet (PBUH) and the Prophet (PBUH) threw him on the ground. Rukanah said: I heard the Prophet (PBUH) say: The difference between us and the polytheists is that we wear turbans over caps.