It was narrated that 'Umar bin Khattab said:
"The Messenger of Allah saw a man performing ablution and he missed a spot the size of a fingernail on his foot. He commanded him to repeat the ablution and his prayer, so he did." (ID)
English reference (ID): Vol. 1, Book 1, Hadith 666 (ID)
Arabic reference (ID): Book 1, Hadith 711 (ID)