It was narrated that 'Ali said:
"A man came to the Prophet and said: 'I bathed to cleanse myself from sexual impurity, and I prayed Fajr, then I noticed a spot the size of a fingernail that the water did not reach.' The Messenger of Allah said: 'If you had wiped it that would have been sufficient for you.'" (DE)
English reference (DE): Vol. 1, Book 1, Hadith 664 (DE)
Arabic reference (DE): Book 1, Hadith 709 (DE)