Why did the canonization of Sunni hadith collections take place gradually?

The canonization of Sunni hadith collections was a gradual process driven by the need to address challenges in verifying the authenticity of reports attributed to the Prophet Muhammad. Following the Prophet's death, hadith transmission was initially informal and localized, relying on oral relaying. As the Islamic empire expanded, the proliferation of reports—including fabrications intended to support personal or political views—necessitated more rigorous authentication methods.12

This evolution occurred in several stages:

• Development of Verification: Scholars developed the isnad system, a chain of narrators used to trace and verify the pedigree of each report.23
• Systematic Compilation: Early efforts included musnad collections, organized by the names of the Companions who transmitted them, and muṣannaf works, which grouped reports by legal topics to assist in scholarly debates.13
• Refinement and Selection: By the 9th century, compilers like al-Bukhārī and Muslim ibn al-Ḥajjāj applied stringent criteria to select only the most sound reports, eventually leading to the recognition of al-kutub al-sittah ("the six books") as the most authoritative collections.1

Even after these works gained prominence, canonization remained relative, as ongoing scholarly analysis continued to evaluate the accuracy of individual hadiths.1