Musnad Ahmad 26:271

'Abd al-Samad narrated to us, Hammad narrated to us, from 'Asim, from Zirr, from Ibn Mas'ud, that it was said to the Messenger of Allah (peace and blessings of Allah be upon him): "How will you recognize those of your nation who have not seen you?" He said: "They will be with white foreheads and white limbs, having white marks from the traces of ablution."