Musnad Ahmad 28:459

Yazid narrated to us, Muhammad ibn Ishaq informed us, from ‘Amr ibn Shu‘ayb, from his father, from his grandfather, who said: "The Messenger of Allah (may Allah bless him and grant him peace) forbade the plucking of gray hair," and he said: "It is the light of the believer." And he said: "No man grows a gray hair in Islam, except that Allah raises him a degree by it, erases a sin from him by it, and writes a good deed for him by it." And the Messenger of Allah (may Allah bless him and grant him peace) said: "He is not of us who does not honor our elder and show mercy to our younger."