The book known to the canon descends from a larger one. An-Nasa'i first compiled al-Sunan al-Kubra, the Great Sunan, a comprehensive gathering of legal traditions with their variant chains; asked, in the traditional account, whether everything in it was sound, he answered by producing a selection purged of the reports he judged defective, al-Mujtaba, the Chosen, and it is this Small Sunan that took the canonical place. His criteria for transmitters were famously severe.
Later critics observed that an-Nasa'i would abandon a narrator on grounds gentler judges excused, and that his standard for accepting a man approached the standard of the Sahih compilers; the tradition preserves his refusal to transmit from narrators others tolerated. The result is a collection prized for the fineness of its chains and for its craft in exposing hidden defects, since an-Nasa'i often sets variant transmissions of a report side by side precisely to show where a narrator erred, making parts of the book a practical course in criticism. Its coverage tilts heavily toward ritual: the chapters on purification and on prayer, its times, postures, recitations and lapses, are the most detailed of the six books, which made the Sunan a first reference for the law of worship in particular. Roughly 5,700 numbered reports stand in the Mujtaba, against more than double that in the Kubra.