"I heard Jabir bin Samurah being asked about the gray hairs of the Prophet [SAW]. He said: 'If he put oil on his head they could not be seen, but if he did not put oil on his head, they could be seen.'"  

عَنْ شَيْبِ رَسُولِ اللَّهِ ﷺ قَالَ كَانَ إِذَا ادُّهِنَ رَأْسُهُ لَمْ يُرَ مِنْهُ وَإِذَا لَمْ يُدَّهَنْ رُئِيَ مِنْهُ