(4) باب الوضوء
Narrated Anas (rad):
The Prophet (ﷺ) saw a man on whose foot appeared a portion like the size of a nail which was not touched by water. He then said, "Go back and perform your Wudu properly." [Reported by Abu Da'ud and An-Nasa'i].
Reference: Bulugh al-Maram 55
In-book reference: Book 1, Hadith 64
English translation: Book 1, Hadith 60