“then break your fast.”1 This ḥadīth is established in the Ṣaḥīḥ and proves that fasting on a Saturday is permissible and carries no blame. On this basis some scholars declared the report forbidding Saturday fasts to be irregular, because it conflicts with a narration of greater authority. Other scholars replied that there is no conflict, for the two reports can be reconciled—and whenever reconciliation is possible, there is neither contradiction nor irregularity. The harmonization is to say that the prohibition concerns singling Saturday out; that is, one is forbidden to fast Saturday on its own. If, however, he fasts it together with Friday or with Sunday, there is no objection at all. It is well known that once reconciliation is possible there is no contradiction and no irregularity.

Another type of irregularity is to contradict what is necessarily known in the religion.
An example is the narration in Ṣaḥīḥ al-Bukhārī that reads: “After the people of this world have entered it, there will still remain space in Hell, so Allah will bring forth a people for it and admit them into the Fire.”2
Although the chain of this report is uninterrupted, it is irregular because it opposes what is necessarily known: Allah does not wrong anyone. In reality the wording was inverted on the transmitter. The correct version is: “There will still remain space in Paradise after the people of this world have entered it, so Allah will bring forth a people for it and admit them into Paradise.” This is a grace that involves no injustice, whereas the first wording entails injustice.

In all cases, for a ḥadīth to be authentic it must not be irregular.
If a single trustworthy and upright narrator relates a ḥadīth one way, and then two narrators of equal probity relate it in a form that contradicts the first, what do we say about the initial version?
We say: the first narration is irregular and, therefore, not authentic—even though it was transmitted by an upright narrator.