Sahih Muslim Book 002, Hadith Number 0481.
Chapter : It is commendable to include forehead in washing the face and elbow and ankle in washing the hands and feet, while performing ablution.
Hudhaifa reported: The Messenger of Allah (may peace be upon him) said: My Cistern is bigger than the space between Aila and Aden. By Him in Whose Hand is my life, I will drive away persons (from it) just as a person drives away unknown camels from his cistern. They (the companions) said: Messenger of Allah, would you recognise us? He said: Yes, you would come to me with white faces, and white hands and feet on account of the traces of ablution. None but you would have (this mark).