It was narrated from ‘Umar Ibn Al Khaṭṭāb (may Allāh be pleased with him) that a man performed wuḍū (ablution) and left a nail sized area on his foot. Then the Prophet (May Allāh praise him and Peace be upon him) saw and said “Go back and perform your wuḍū properly”, so he went back, and he prayed after that [Ṣaḥīḥ Muslim 243].