Musnad Ahmad 28:196

Isma'il narrated to us, Laith narrated to us, from 'Amr ibn Shu'ayb, from his father, from his grandfather, who said: 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. There is no Muslim who grows a gray hair in Islam except that a good deed is written for him by it, and he is raised a degree by it, or a sin is removed from him by it."