Sayyiduna Abu Umamah (may Allah be pleased with him) narrates: The Prophet (peace and blessings be upon him) said: "On the Day of Resurrection I will recognise every single member of my Ummah. The people said: O Messenger of Allah! Even the one whom you have seen, and also the one whom you (peace and blessings be upon him) have not seen? The Prophet (peace and blessings be upon him) said: The one I have seen and the one I have not seen (I will recognise both kinds of people), because they will have shining foreheads from the traces of ablution."
Imam Ahmad and Imam al-Tabarani (in al-Mu'jam al-Kabir) recorded this report, and its narrators have been declared trustworthy.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1137