Hazrat Anas (Radiallahu anhu) reports, "Sayyidina Rasulullah (Sallallahu alaihe wasallam) often rubbed oil in his head and also often combed his beard. He put a cloth over his head, which became like an oil cloth due to the frequent use of oil." (ID)
English reference (ID): Book 4, Hadith 32 (ID)
Arabic reference (ID): Book 4, Hadith 33 (ID)