The word sahih means sound, and it names a standard before it names a book. By al-Bukhari's century, hundreds of thousands of reports about the Prophet circulated, ranging from the meticulously transmitted to the invented, and the science of hadith criticism had developed to sort them: every report travels with its isnad, the chain of people who passed it on, and critics assessed each link for honesty, accuracy of memory and the real possibility that transmitter met transmitter. Al-Bukhari's innovation, according to the tradition's own account, was to compile a book restricted to reports that survived the strictest form of this test, requiring evidence that consecutive transmitters had actually met rather than merely overlapped in time.
Tradition relates that he sifted some six hundred thousand reports over sixteen years of travel from Bukhara across Khurasan, Iraq, the Hijaz, Syria and Egypt, and prayed before including each tradition. The collection contains roughly 7,275 reports with repetitions, since a single tradition may appear under several legal headings, and about 2,602 distinct ones. Later criticism did not treat the book as beyond question, and a small number of its reports and transmitters were debated by scholars such as al-Daraqutni, but the debates confirmed rather than dented the consensus that formed around it: among books of hadith, this one set the bar.