Musnad Ahmad 28:511

Ishaq ibn Isa narrated to us, Abd al-Rahman ibn Abi al-Zinad narrated to us, from Abd al-Rahman ibn al-Harith, from Amr ibn Shu'ayb—if Allah wills—from his father, from his grandfather, that the Messenger of Allah (peace and blessings of Allah be upon him) forbade plucking out gray hair. He said: "It is the light of Islam."