The biographical tradition around al-Bukhari makes the scale of his search for hadith almost a genre unto itself. He is said to have travelled to the major centres of learning across the ninth-century Islamic world, Balkh, Merv, Baghdad, Basra, Kufa, Mecca, Medina, Damascus and Fustat among them, seeking out scholars who could pass on reports of the Prophet's words and actions with an unbroken chain of named transmitters back to someone who had heard them directly. The scale attributed to him in later sources, several hundred thousand hadith examined, should be read as a measure of reputation as much as an exact count, since a single report repeated through many different chains was counted separately each time.
What made Sahih al-Bukhari distinctive was the severity of its filter. He is reported to have required not only an unbroken chain but positive evidence that each narrator in it had actually met the person from whom they claimed to have heard the report, a stricter test than some contemporaries applied, and to have excluded a report entirely if any narrator in its chain fell short on memory or character by his standard. The result, a selection of roughly seven thousand hadith counting repetitions, from a search on that scale, is the reason Sunni scholarship treats his selectivity itself as evidence of reliability: what survived his filter had already passed through the harshest test available in his century.
He organised the surviving reports by subject, prefacing chapters with terse headings that themselves became objects of commentary, since the placement of a hadith under one heading rather than another was read as an argument about law. Centuries of commentary literature grew up around the book for exactly this reason, reading his silences and arrangements as carefully as his text.