How did hadiths become a major source of Islamic legal rulings?

Hadiths became a major source of Islamic legal rulings through a gradual process of collection, classification, and the development of a systematic methodology for legal interpretation.

Initially, early Islamic legal schools relied on regional traditions and the practices of their respective communities. However, as the Islamic empire expanded, the systematic collection of reports concerning the sayings and actions of the Prophet Muhammad—the Hadith—challenged these local customs.

The transition toward prioritizing Hadith was largely driven by the legal scholar Abū ʿAbd Allāh al-Shāfiʿī. He rejected the authority of regional traditions and argued that the Qurʾān and the Prophetic example (Sunnah), as preserved in authentic Hadith, should be the primary sources of law. To address the challenge of fabricated reports, scholars developed ʿilm al-ḥadīth, a rigorous science of authentication. This allowed jurists to filter reports and establish a reliable basis for fiqh (jurisprudence). Ultimately, these efforts led to the canonization of authoritative collections, such as the Sunni al-kutub al-sittah, which solidified the role of Hadith as a foundational pillar of Sharīʿah second only to the Qurʾān.123