Musnad Ahmad 2:53

Musa ibn Dawud narrated to us, Ibn Lahi'ah narrated to us, from Abu al-Zubayr, from Jabir: That Umar ibn al-Khattab informed him that he saw a man perform ablution for prayer, and he left a spot the size of a fingernail on the top of his foot. The Prophet, peace and blessings of Allah be upon him, saw him and said: "Go back and perform your ablution well." So he went back, performed ablution, and then prayed.