When he oiled his head nothing was seen (as a mark of old age) and when he did not apply oil something (of the old age) became visible. (Using translation from Muslim 2344a)  

عَنْ شَيْبِ النَّبِيِّ ﷺ قَالَ «كَانَ إِذَا دَهَنَ رَأْسَهُ لَمْ يُرَ مِنْهُ وَإِذَا لَمْ يَدَّهِنْ رُئِيَ مِنْهُ»