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