´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."
Hadith Referenceسنن ابن ماجه / أبواب التيمم / 666
Hadith Gradingالألبانی:صحيح | زبیر علی زئی:صحيح مسلم