Musnad Abu Ya'la al-Mawsili 115:103

Harun ibn Ma'ruf narrated to me, Ibn Wahb narrated to us, he said: Jarir ibn Hazim narrated to me that he heard Qatadah ibn Di'amah, he said: Anas ibn Malik narrated to us that a man came to the Prophet, may Allah bless him and grant him peace, having performed ablution, but he left [a spot] on his feet like the place of a fingernail. The Messenger of Allah, may Allah bless him and grant him peace, said: "Go back and perform your ablution well."