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. (ID)
English reference (ID): Book 19, Hadith 119 (ID)
Arabic reference (ID): Book 20, Hadith 126 (ID)