محمد بن إسماعيل البخاري
The "science of hadith" is the process that Muslim scholars use to evaluate hadith. The classification of Hadith into Sahih (sound), Hasan (good) and Da'if (weak) was firmly established by Ali ibn al-Madini (161–234 AH). Later, al-Madini's student Muhammad al-Bukhari (810–870) authored a collection that he believed contained only Sahih hadith, which is now known as the "Sahih Bukhari". Al-Bukhari's historical methods of testing hadiths and isnads is seen as the beginning of the method of citation and a precursor to the scientific method. I. A. Ahmad writes: