As time passed, more reporters were involved in each isnad, and so the situation demanded strict discipline in the acceptance of ahadith; the rules regulating this discipline are known as Mustalah al-Hadith (the Classification of Hadith).
Amongst the early traditionists (muhaddithin, scholars of Hadith), the rules and criteria governing their study of Hadith were meticulous but some of their terminology varied from person to person, and their principles began to be systematically written down, but scattered amongst various books, e.g. in Al-Risalah of al- Shafi'i (d. 204), the Introduction to the Sahih of Muslim (d. 261) and the Jami' of al-Tirmidhi (d. 279); many of the criteria of early traditionists, e.g. al-Bukhari, were deduced by later scholars from a careful study of which reporters or isnads were accepted and rejected by them.
One of the earliest writings to attempt to cover Mustalah comprehensively, using standard (i.e. generally-accepted) terminology, was the work by al-Ramahurmuzi (d. 360). The next major contribution was Ma'rifah 'Ulum al-Hadith by al- Hakim (d. 405), which covered fifty classifications of Hadith, but still left some points untouched; Abu Nu'aim al-Isbahani (d. 430) completed some of the missing parts to this work. After that came Al-Kifayah fi 'Ilm al- Riwayah of al-Khatib al-Baghdadi (d. 463) and another work on the manner of teaching and studying Hadith; later scholars were considered to be greatly indebted to al-Khatib's work.
After further contributions by Qadi 'Iyad al- Yahsubi (d. 544) and Abu Hafs al-Mayanji (d. 580) among others, came the work which, although modest in size, was so comprehensive in its excellent treatment of the subject that it came to be the standard reference for thousands of scholars and students of Hadith to come, over many centuries until the present day: 'Ulum al- Hadith of Abu 'Amr 'Uthman Ibn al-Salah (d. 643), commonly known as Muqaddimah Ibn al-Salah, compiled while he taught in the Dar al-Hadith of several cities in Syria. Some of the numerous later works based on that of Ibn al-Salah are:
- An abridgement of Muqaddimah, Al-Irshad by al- Nawawi (d. 676), which he later summarised in his Taqrib; al-Suyuti (d. 911) compiled a valuable commentary on the latter entitled Tadrib al-Rawi.
- Ikhtisar 'Ulum al-Hadith of Ibn Kathir (d. 774), Al-Khulasah of al-Tibi (d. 743), Al- Minhal of Badr al-Din b. Jama'ah (d. 733), Al- Muqni' of Ibn al-Mulaqqin (d. 802) and Mahasin al-Istilah of al-Balqini (d. 805), all of which are abridgements of Muqaddimah Ibn al- Salah.
- Al-Nukat of al-Zarkashi (d. 794), Al-Taqyid wa 'l-Idah of al-'Iraqi (d. 806) and Al-Nukat of Ibn Hajar al-'Asqalani (d. 852), all of which are further notes on the points made by Ibn al- Salah.
- Alfiyyah al-Hadith of al-'Iraqi, a rewriting of Muqaddimah in the form of a lengthy poem, which became the subject of several commentaries, including two (one long, one short) by the author himself, Fath al-Mughith of al-Sakhawi (d. 903), Qatar al-Durar of al- Suyuti and Fath al-Baqi of Shaykh Zakariyyah al-Ansari (d. 928).
Other notable treatises on Mustalah include:
- Al-Iqtirah of Ibn Daqiq al-'Id (d. 702). Tanqih al-Anzar of Muhammad b. Ibrahim al- Wazir (d. 840), the subject of a commentary by al-Amir al-San'ani (d. 1182).
- Nukhbah al-Fikr of Ibn Hajar al-'Asqalani, again the subject of several commentaries, including one by the author himself, one by his son Muhammad, and those of 'Ali al-Qari (d. 1014), 'Abd al-Ra'uf al-Munawi (d. 1031) and Muhammad b. 'Abd al-Hadi al-Sindi (d. 1138). Among those who rephrased the Nukhbah in poetic form are al-Tufi (d. 893) and al- Amir al-San'ani.
- Alfiyyah al-Hadith of al-Suyuti, the most comprehensive poetic work in the field. Al-Manzumah of al-Baiquni, which was expanded upon by, amongst others, al-Zurqani (d. 1122) and Nawab Siddiq Hasan Khan (d. 1307). Qawa'id al-Tahdith of Jamal al-Din al-Qasimi (d. 1332).
- Taujih al-Nazar of Tahir al-Jaza'iri (d. 1338), a summary of al-Hakim's Ma'rifah.