Musnad Abu Ya'la al-Mawsili 152:232

Abu Khaythama narrated to us, Yazid bin Harun narrated to us, Hammad bin Salama narrated to us, from Asim, from Zirr, from Abdullah, who said: It was said, "O Messenger of Allah, how will you recognize those of your nation whom you have not seen on the Day of Resurrection?" He said, "They will be 'ghurr' (having white foreheads) and 'muhajjalun' (having white limbs), with white marks from the traces of ablution."