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.'"
Sunan Ibn Majah Book 1, Hadith 399
← Previous Hadith "A man came to the Prophet and said: 'I bathed to cleanse myself from sexual impurity, and I prayed Fajr, then I noticed a spot the size of a fingernail that the water did not reach.' The Messenger of Allah said: 'If you had wiped it that would have been sufficient for you.'"

Sunan Ibn Majah Book 1, Hadith 398 Next Hadith → "The Messenger of Allah saw a man performing ablution and he missed a spot the size of a fingernail on his foot. He commanded him to repeat the ablution and his prayer, so he did."

Sunan Ibn Majah Book 1, Hadith 400