Moreover, Abū Dāwūd al-Sijistānī—may Allah have mercy on him—authored a separate work on the singular reports maintained by the scholars of the various regional centers. In it he listed the Sunan that are unique to each locality, reports whose isnād is not found with scholars of any other city—such as Medina, Mecca, al-Ṭāʾif, Damascus, Ḥoms, Kūfa, Baṣra, and others. There are, however, additional causes beyond these.

Fourth cause:
A scholar may stipulate, for accepting a solitary report from an upright, reliable memoriser, conditions that other scholars do not require. Thus, some insist that the ḥadith be compared with the Qur’an and the Sunnah; others demand that the transmitter be a jurist whenever the report conflicts with established analogical principles; yet others require that the ḥadith be widespread and well known if it concerns matters of common public involvement. Other such conditions exist as well, each discussed in its proper place.

Fifth cause:
A ḥadith may have reached a scholar and been confirmed as authentic, yet he later forgets it.

This happens with both the Qur’an and the Sunnah. An instructive example is the well-known report from ʿUmar (may Allah be pleased with him): he was asked about a man who falls into a state of major ritual impurity while travelling and finds no water. ʿUmar replied,