Sahih Muslim Book 2, Hadith Number 480.

Chapter : It is commendable to include forehead in washing the face and elbow and ankle in washing the hands and feet, while performing ablution.

Abu Huraira reported 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.