Sahih Muslim Book 3, Hadith Number 703.

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.

Abu Huraira reported: The Messenger of Allah (may peace be upon him) said: If any one of you has pain in his abdomen, but is doubtful whether or not anything has issued from him, be should not leave the mosque unless he hears a sound or perceives a smell.