Hadith (traditions of the Prophet) are the second source of Islamic law and theology after the Quran. Hadith science — the systematic methodology for evaluating the authenticity of hadith — is one of the great intellectual achievements of early Islamic civilization. The core method: isnad criticism — evaluating the chain of transmitters (isnad) back to the Prophet to determine whether each transmitter was reliable, of good memory, and actually met the next link in the chain. The Six Books of Sunni hadith are the canonical collections: Sahih al-Bukhari and Sahih Muslim are the two highest-regarded (‘the two Sahihs’); followed by Abu Dawud, al-Tirmidhi, al-Nasa’i, and Ibn Majah. Imam al-Bukhari (810–870 AD) reportedly examined 600,000 hadith and accepted only ~7,400 as authentic. Hadith science created a vast biographical literature on the reliability of thousands of narrators — arguably the first systematic approach to historical source criticism in any civilization.