Musnad Ahmad 937:49

Abdullah ibn Idris narrated to us, he said: I heard al-Ajalh, from Ibn Buraydah, from Abu al-Aswad al-Du'ali, from Abu Dharr, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, among the best of what you change gray hair with is henna and katam."