Jaabir bin Samurah R.A. was asked a question regarding the white hair of Rasulullah (Sallallahu alaihe wasallam). He replied:
"When Rasulullah (Sallallahu alaihe wasallam) oiled his hair, it (white hair) did not show. When he did not oil it, it showed".
English reference: Book 5, Hadith 38
Arabic reference: Book 5, Hadith 39