Musnad Ahmad 2:72

Al-Hasan narrated to us, Ibn Lahi'ah narrated to us, Abu al-Zubayr narrated to us, from Jabir, that Umar ibn al-Khattab informed him that the Messenger of Allah (peace and blessings of Allah be upon him) saw a man who performed ablution for the noon prayer, but he left a spot the size of a fingernail on the top of his foot [unwashed]. The Messenger of Allah (peace and blessings of Allah be upon him) saw him and said: "Go back and perform your ablution well." So he went back and performed ablution, then he prayed.