Sayyiduna Anas (may Allah be pleased with him) relates that a man came to the Prophet (peace and blessings be upon him). He had performed ablution and left a spot the size of a fingernail dry on his foot. the Messenger of Allah (peace and blessings be upon him) said: "Go back and perform your ablution well."
(b) Imam Abu Dawud (may Allah have mercy on him) says that this hadith is not well known, and only Ibn Wahb narrated it from Jarir bin Hazim.
(c) Imam Abu Dawud (may Allah have mercy on him) says: Al-Hasan (al-Basri) narrated it from the Prophet (peace and blessings be upon him) with the meaning of Qatadah's hadith, and this is mursal.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 393