(10) باب وُجُوبِ اسْتِيعَابِ جَمِيعِ أَجْزَاءِ مَحَلِّ الطَّهَارَةِ
Jabir reported:
'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 reference: Book 2, Hadith 43
USC-MSA web (English) reference(deprecated numbering scheme): Book 2, Hadith 474