Musnad Ahmad 28:484

Abu Bakr al-Hanafi narrated to us, 'Abd al-Hamid ibn Ja'far narrated to us, from 'Amr ibn Shu'ayb, from his father, from his grandfather, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Do not pluck out gray hairs, for it is the light of the Muslim. Whoever grows a gray hair in Islam, Allah writes for him a good deed because of it, erases a sin from him because of it, and raises him a degree because of it."