Musnad Ahmad 937:200

Ibn Numayr narrated to us, Al-Ajalih narrated to us, from Abdullah 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, the best of what gray hair is changed with is henna and katam."