Musannaf Ibn Abi Shayba 1:446

'Abd al-Rahim ibn Sulayman narrated to us, from Hajjaj, from 'Ata', from 'Ubayd ibn 'Umayr, that 'Umar ibn al-Khattab saw a man who had a spot on his foot that water had not reached when he performed purification. 'Umar said to him, "With this ablution you attend the prayer," and he ordered him to wash the spot and repeat the prayer.