Musnad Abu Ya'la al-Mawsili 43:1

Abu Ya'la informed us, Abu Kurayb narrated to us, Muhammad ibn Rabi'ah narrated to us, he said: I met in Makkah a man from the people of Ashkelon who was called Abu al-Hasan. He narrated to me from Abu Ja'far ibn Muhammad ibn Rukanah, from his father, that he wrestled with the Prophet, peace and blessings of Allah be upon him. Rukanah said: I heard the Prophet, peace and blessings of Allah be upon him, saying: "The distinction between us and the polytheists is the turbans over the caps."