سنن ابن ماجه

'Abdullah bin Mas'ud said: "It was said: 'O Messenger of Allah, how will you recognize those whom you have not seen of your Ummah?' He said: 'From the blazes of their foreheads and feet, like horses with black and white traces (which make them distinct from others) which are the traces of ablution.'" (Hasan) Another chain with similar wording.