Sahih Muslim Book 003, Hadith Number 0702.
Chapter : A man who is sure of his purification, but entertains doubt of anything breaking it, can safely offer prayer without performing a new ablution.
'Abbad b. Tamim reported from his uncle that a person made a complaint to the Apostle (may peace be upon him) that he entertained (doubt) as it something had happened to him breaking his ablution. He (the Holy Prophet) said: He should not return (from prayer) unless he hears a sound or perceives a smell (of passing wind). Abu Bakr and Zuhair b. Harb have pointed out in their narrations that it was 'Abdullah b. Zaid.