Ka'b b. Murra reported God’s messenger as saying, “If anyone grows a grey hair in Islam it will be a light for him on the day of resurrection.” Tirmidhi and Nasa’i transmitted it.

وَعَنْ كَعْبِ بْنِ مُرَّةَ عَنْ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: «مَنْ شَابَ شَيْبَةً فِي الْإِسْلَامِ كَانَتْ لَهُ نُورًا يَوْمَ القيامةِ» . رَوَاهُ التِّرْمِذِيّ وَالنَّسَائِيّ