It was narrated from Anas that:
A man came to the Prophet; he had performed ablution and has missed a spot the size of a fingernail where water had not reached. The Prophet said to him: 'Go back and perform ablution properly.'" (ID)
English reference (ID): Vol. 1, Book 1, Hadith 665 (ID)
Arabic reference (ID): Book 1, Hadith 710 (ID)