Musnad Ahmad 26:779

Affan narrated to us, Hammad ibn Salamah narrated to us, Asim ibn Bahdalah informed us, from Zirr ibn Hubaysh, from Ibn Mas'ud, that they said: "O Messenger of Allah, how will you recognize those of your nation whom you have not seen?" He said: "They will have white foreheads and white limbs from the traces of ablution."