Musnad Abu Ya'la al-Mawsili 151:82

Kamil ibn Talhah narrated to us, Hammad ibn Salamah narrated to us, from ‘Asim ibn Bahdalah, from Zirr ibn Hubaysh, from ‘Abdullah 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."