Umar b. Khattab said that a person performed ablution and left a small part equal to the space of a nail (unwashed). The Apostle of Allah (ﷺ) saw that and said: Go back and perform ablution well. He then went back (performed ablution well) and offered the prayer.
Reference: Sahih Muslim 243 In-book : Book 2, Hadith 43 USC-MSA web (English) : Book 2, Hadith 474 (deprecated)
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