The Quran is, by universal Muslim agreement, the first source of Islamic law, but it does not by itself supply the detail a functioning legal and devotional system needs: the exact form of the five daily prayers, the precise rates owed in almsgiving, the particulars of marriage and inheritance are worked out from a second source, the Sunna, the Prophet Muhammad's own recorded example. The Sunna reaches later generations through hadith, individual reports of what the Prophet said, did or approved of, each one carried by a chain of named transmitters, an isnad, stretching back to someone who witnessed it directly. Because the reliability of a hadith rests entirely on the reliability of that chain, Islamic scholarship developed an entire discipline, the science of hadith criticism, to grade each report by how trustworthy and how unbroken its chain of transmitters is, sorting reports into ranks from sound, sahih, down through acceptable, hasan, to weak, da'if, and lower still. Which specific collections of hadith a tradition treats as most authoritative differs across the branches of Islam, but the underlying claim, that the Sunna stands as a binding source of law second only to the Quran itself, is shared ground across nearly the entire faith.