Abu Dawud
Purification (Kitab Al-Taharah)
Book Number: 001 / Hadith Number: 176
Narated By 'Abbad b. Tamim from his uncle : A person made a complaint to the Prophet (pbuh) that he entertained (doubt) as if something had happened to him which had rendered his ablution invalid. He (the Prophet) said: He should nor cease (to pray) unless he hears a sound or perceives a smell (of passing wind).