Musnad Ahmad 28:199

Yahya ibn Sa'id narrated to us, from Ibn 'Ajlan, he said: 'Amr ibn Shu'ayb narrated to me, from his father, from his grandfather, from the Prophet (peace and blessings of Allah be upon him), who said: "Do not pluck out gray hairs, for there is no servant who grows a gray hair in Islam, except that Allah writes for him a good deed because of it, and removes from him a sin because of it."