Abu Dawud
Purification (Kitab Al-Taharah)
Book Number: 001 / Hadith Number: 177
Narated By Abu Hurairah : The Messenger of Allah (pbuh) said: If anyone of you offers prayer and feels a movement between his paddocks, but is doubtful whether or not his ablution broke, he should not cease praying unless he hears a sound or perceives a smell.