Musnad Ahmad 1118:18

Qutaybah ibn Sa'id narrated to us, Ibn Lahi'ah narrated to us, from Yazid ibn Abi Habib, from 'Abd al-'Aziz ibn Abi al-Sa'bah, from Hanash, from Fudalah ibn 'Ubayd, that the Prophet, peace and blessings of Allah be upon him, said: "Whoever develops a gray hair in the way of Allah, it will be a light for him on the Day of Resurrection." A man said at that: "But there are men who pluck out their gray hairs." The Messenger of Allah, peace and blessings of Allah be upon him, said: "Whoever wishes, let him pluck out his light."