Abu Dawud
Purification (Kitab Al-Taharah)
Book Number: 001 / Hadith Number: 175
Narated By Some Companions of the Prophet : The Prophet (pbuh) saw a person offering prayer, and on the back of his foot a small part equal to the space of a dirham remained unwashed; the water did not reach it. The Prophet (pbuh) commanded him to repeat the ablution and prayer.