Musnad Ahmad 30:2086

Ali ibn Ishaq narrated to us, Abdullah narrated to us, Yunus narrated to us, from al-Zuhri, Abu Salamah ibn Abd al-Rahman informed me, that Abu Hurairah said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, the Jews and the Christians do not dye [their hair/beards], so differ from them."