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." (ES)
English reference (ES): Vol. 1, Book 1, Hadith 666 (ES)
Arabic reference (ES): Book 1, Hadith 711 (ES)