Mustalah books speak of a number of classes of hadith in accordance with their status. The following classifications can be made, each of which is explained later:
1. According to the reference to a particular authority, e.g. the Prophet (SAS), a Companion, or a Successor; such ahadith are called marfu' (elevated), mauquf (delayed) and maqtu' (severed) respectively .
2. According to the nature of the chain of reporters, i.e. whether interrupted or uninterrupted, e.g. musnad (supported), muttasil (continuous), munqati'' (broken), mu'allaq (suspended), mu'dal (perplexing) and mursal (loose).
3. According to the number of reporters involved in each isnad, e.g. mutawatir (consecutive) and ahad (isolated), the latter being divided into gharib (rare), 'aziz (scarce), and mash-hur (widespread) .
4. According to the way in which a saying has been reported such as using the words 'an ( - "on the authority of"), haddathana ( - "he narrated to us"), akhbarana ( - "he informed us") or sami'tu ( - "I heard"). In this category falls the discussion about mudallas (concealed) and musalsal (connected) ahadith.
5. According to the nature of the matn and isnad, e.g. an addition by a reliable reporter, known as ziyadah thiqa, or opposition by a lesser authority to a more reliable one, known as shadh (aloof). In some cases a text containing a vulgar expression, unreasonable remark or an apparently erroneous statement is rejected by the traditionists outright without consideration of the isnad. Such a hadith is known as munkar (denounced). If an expression or statement is proved to be an addition by a reporter to the text, it is declared as mudraj (added).
6. According to a hidden defect found in the isnad or text of a hadith. Although it could be included in some of the previous categories, hadith mu'allal (defective hadith) is worthy to be explained separately. The defect can be caused in many ways; e.g. two types of hadith mu'allal are known as maqlub (overturned) and mudtarib (shaky).
7. According to the reliability and memory of the reporters; the final verdict on a hadith depends mainly on this classification: verdicts such as sahih (sound), hasan (good), da'if (weak) and maudu' (fabricated) rest mainly upon the nature of the reporters in the isnad.
Musatalah al-hadith is strongly associated with Rijal al-hadith (the study of the reporters of hadith). In scrutinising the reporters of a hadith, authenticating or disparaging remarks made by recognised experts, whether among the Successors or those after them, were found to be of great help. The earliest remarks cited in the books of Rijal go back to a host of Successors and those after during the first three centuries of Islam. A list of such names is provided by the author in his thesis, Criticism of Hadith among Muslims with reference to Sunan lbn Maja, at the end of chapters IV, V and VI. Among the earliest available works in this field are Tarikh of Ibn Ma'in (d. 233), Tabaqat of Khalifa b. Khayyat (d. 240), Tarikh of Bukhari (d. 256), Kitab al-Jarh wa al-Ta'dil of Ibn Abi Hatim (d. 327) and Tabaqat of Muhammad b. Sa'd al-Zuhri (d. 320).
A number of traditionists made efforts specifically for the gathering of information about the reporters of the five famous collections of hadith: those of Bukhari (d. 256), Muslim (d. 261), Abu Dawud (d. 275), Tirmidhi (d. 279) and Nasa'i (d. 303), giving authenticating and disparaging remarks in detail. The first major such work to include also the reporters of Ibn Maja (d. 273) is the ten-volume collection of al-Hafiz 'Abd al-Ghani al-Maqdisi (d. 600), known as Al-Kamal fi Asma' al-Rijal. Later, Jamal al-Din 'Abd al-Hajjaj Yusuf b. 'Abd al-Rahman al-Mizzi (d. 742) prepared an edited and abridged version of this work, but made a number of additions and punctuation of the names by names, places and countries of origin of the reporters. He named it Tahdhib al-Kamal fi Asma' al-Rijal and produced it in twelve volumes. Further, one of al-Mizzi's gifted pupils, Shams al-Din Abu 'Abdullah Muhammad b. Ahmad b. 'Uthman b. Qa'imaz al-Dhahabi (d. 748), summarised his shaikh's work and produced two abridgements: a longer one called Tadhhib al-Tahdhib and a shorter one called Al-Kashif fi Asma' Rijal al-Kutub al-Sitta.
A similar effort with the work of Mizzi was made by Ibn Hajar (d. 852), who prepared a lengthy but abridged version, with about one-third of the original omitted, entitled Tahdhib al-Tahdhib in twelve shorter volumes. Later, he abridged this further to a relatively humble two-volume work called Taqrib al-Tahdhib.
The work of Dhahabi was not left unedited; Khazraji (Saif al-Din Ahmad b. 'Abdullah, d. after 923) summarised it and also made valuable additions, producing his Khulasa.
A number of similar works deal with either trustworthy authorities, e.g. Kitab al-Thiqat by 'Ijli (d. 261) and Tadhkira al-Huffaz by Dhahabi, or with disparaged authorities, e.g. Kitab al-Du'afa' wa al-Matrukin by Nasa'i and Kitab al-Majruhin by Muhammad b. Hibban al-Busti (d. 354).
Two more works in this field, which include a large number of reporters, both authenticated and disparaged, are Mizan al-l'tidal of Dhahabi and Lisan al-Mizan of Ibn Hajar.