Abu Dawud Book 1, Hadith Number 175.

Abu Dawud Book 1. Purification

Chapter : Leaving any spot dry in ablution.

Narated By Some Companions of the Prophet : The Prophet (PBUH) saw a person offering prayer, and on the back of his foot a small part equal to the space of a dirham remained unwashed; the water did not reach it. The Prophet (PBUH) commanded him to repeat the ablution and prayer.

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