Abu Dawud Book 1, Hadith Number 177.

Chapter : A man who is sure of purification but doubts that something has rendered it invalid.

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.