Narration of Umar b. al-Khattab radiallaahu ‘anhu that a man had performed ablution, he left part on his foot unwashed (size of a fingernail) Prophet sallahu alayhi wassallam commanded him to perform your Wudhu correctly… (Saheeh Muslim)
In another narration, Prophet saw a man praying, back of his foot (size of a dirham (water had not reached) Prophet sallahu alayhi wassallam commanded him to do the Wudhu (ablution) again. (Saheeh in Sunan Abu Dawud and Irwaa al Ghalil)
*Note* : He was commanded to do the whole Wudhu because there was break in Continuity as Prophet sallahu alayhi wassallam did not say wash only that part.
[Taken from the lessons on “Conditions, Pillars and Obligation of Salah – Shaykh ul Islaam Muhammad bin Abdil Wahhab رحمه الله Explanation by Shaykh Abdul Muhsin al-Abbaad حفظه الله Taught by: Abdulilah Lahmami]