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