Anas bin Malik reported,

A man came to the Prophet ﷺ after performing ablution, but he had left a spot the size of a fingernail untouched by water. The Prophet ﷺ said to him, "Go back and perform your ablution properly."

Ibn Majah 665

Sahih

حدثنا حرملة بن يحيى حدثنا عبد الله بن وهب حدثنا جرير بن حازم عن قتادة

عن أنس

أن رجلا أتى النبي ﷺ وقد توضأ وترك موضع الظفر لم يصبه الماء فقال له النبي ﷺ ارجع فأحسن وضوءك

ابن ماجه 665صحيح
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 665
Ibn Majah 665