Sahih Al Bukhari – Book of Ablutions (Wudu')

Volumn 001, Book 004, Hadith Number 139.
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Narated By 'Abbas bin Tamim RA : My uncle asked Allah's Apostle PBUH about a person who imagined to have passed wind during the prayer. Allah' Apostle PBUH replied: "He should not leave his prayers unless he hears sound or smells something."