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.

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."