Umar bin Khattab reported,

The Messenger of Allah ﷺ saw a man who had performed ablution but had left an area the size of a fingernail on his foot dry, so he ordered him to repeat the ablution and the prayer. So the man went back.

Ibn Majah 666

Sahih

حدثنا حرملة بن يحيى حدثنا ابن وهب ح و حدثنا ابن حميد حدثنا زيد بن الحباب قالا حدثنا ابن لهيعة عن أبي الزبير عن جابر

عن عمر بن الخطاب قال

رأى رسول الله ﷺ رجلا توضأ فترك موضع الظفر على قدمه فأمره أن يعيد الوضوء والصلاة قال فرجع

ابن ماجه 666صحيح
Contents
Reference
Commentary
Ayat
Collection
Sunan Ibn Majah
Book
Purification and its sunnahs
Chapter
One who performed wudu but left a spot that water did not reach
Numbering
Darussalam 666
Ibn Majah 666