This hadith forms the basis for one of the major fiqhi guidelines, which is that certainty cannot be dispelled by uncertainty. In it, it says that ‘Abdullah ibn Zayd (may Allah be pleased with him) came to the Prophet (blessings and peace of Allah be upon him) and complained to him that a man may sometimes think that he feels something whilst praying, meaning that he thinks that he has passed wind. But the Prophet (blessings and peace of Allah be upon him) responded by telling him that he should not exit his prayer unless he is certain that he has passed wind, which is when he hears a sound or notices a smell, because he is certain that he was in a state of purity [when he started praying], and that certainty cannot be dispelled because of mere uncertainty; rather he should be certain that his wudu’ has been invalidated and that he has indeed passed wind. Hearing a sound and noticing a smell are mentioned because that is what usually happens when wudu’ is invalidated whilst praying. It cannot be imagined that it would be invalidated in any other way. It is as if he answered the questioner by telling him what he needed to know in most cases, or what usually happens when praying, because urinating, defecating, touching and other things that invalidate wudu’ do not usually happen when one is praying. It was said that what the Prophet (blessings and peace of Allah be upon him) meant is that one should be certain that wudu’ has been invalidated; he did not mean that he should literally have heard it or smelt it, because wind may come out with no sound and no smell, or there may be something wrong with his hearing or sense of smell, so he does not hear the sound or notice the smell. In that case, the person’s wudu’ is invalidated if he is certain that he has passed wind..