Abu Dawud Book 1, Hadith Number 173.

Chapter : Leaving any spot dry in ablution.

Narated By Anas : A person came to the Messenger of Allah (may peace be upon him). He performed ablution and left a small part equal to the space of a nail upon his foot… The Messenger of Allah (may peace be upon him) said to him: Go back and perform ablution well.

Abu Dawud said: This tradition is not known through Jarir b. Hazim. It was transmitted only by Ibn Wahab.

Another version adds the wording: “Go back and perform the ablution well.”