Musannaf Ibn Abi Shayba 1:40

Yazid ibn Harun narrated to us, from Hammad ibn Salamah, from ‘Asim, from Zirr, from ‘Abdullah, who said: I said, "O Messenger of Allah, may Allah bless him and grant him peace, 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."