Hazrat Anas bin Malik radiyallahu anhu says, "Rasoollullah sallallahu alaihe wasallam often wore a cloth on his auspicious head. This cloth, because of its greasiness, looked as if had been oiled. (IT)
English reference (IT): Book 19, Hadith 119 (IT)
Arabic reference (IT): Book 20, Hadith 126 (IT)