Sunan Ibn Majah
The Book of Purification and its Sunnah
كتاب الطهارة وسننها
Chapter 139: One who performed ablution and left a place where water did not reach.
مَنْ تَوَضَّأَ فَتَرَكَ مَوْضِعًا لَمْ يُصِبْهُ الْمَاءُ
Translation
It was narrated that 'Umar bin Khattab said
"The Messenger of Allah saw a man performing ablution and he missed a spot the size of a fingernail on his foot. He commanded him to repeat the ablution and his prayer, so he did."