Sahih Muslim, Chapter: Purification (Kitab Al-Taharah)

the Messenger of Allah (may peace be upon him) said: My people would come to me on the Cistern and I would drive away persons (from it) just as a person drives away other people's camels from his camels. They (the hearers) said: Apostle of Allah, would you recognize us? He replied: Yea, you would have a mark which other people will not have. You would come to me with a white blaze on your foreheads and white marks on your feet because of the traces of ablution. A group among you would be prevented from coming to me, and they would not meet me, and I would say: O my Lord, they are my companions. Upon this an angel would reply to me saying: Do you know what these people did after you.