Musnad Ahmad 30:964

'Abd al-Razzaq narrated to us, Ma'mar and 'Abd al-A'la informed us, from Ma'mar, from al-Zuhri, from Abu Salamah, from Abu Hurayrah, who 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], so differ from them." 'Abd al-Razzaq said in his narration: Al-Zuhri said: "And he commanded the dyes, and the blackest of them is more beloved to us." Ma'mar said: "And al-Zuhri used to dye [his hair] with black."