Sahih Muslim Book 2, Hadith Number 474.

Chapter : It is obligatory to wash all parts of the body necessary for purification.

Jabir reported: ‘Umar b. Khattab said that a person performed ablution and left a small part equal to the space of a nail (unwashed). The Apostle of Allah (may peace be upon him) saw that and said: Go back and perform ablution well. He then went back (performed ablution well) and offered the prayer.