Hadith studies (Arabic: ?ilm al-?ad?th "knowledge of hadith", also science of hadith, or science of hadith criticism) consist of several religious disciplines used in the study and evaluation of the Islamic hadith -- i.e. the record of the words, actions, and the silent approval of the Islamic prophet, Muhammad, by Muslim scholars. The sciences seeks to determine the genuineness (sihha) of ahadith, primarily by attempting to determine whether there are "other identical reports from other transmitters"; the reliability of the transmitters of the report; and "the continuity of the chain of transmission".
It has been described by one hadith specialist, Jalal al-Din al-Suyuti, as the science of the principles by which the conditions of both the sanad, the chain of narration, and the matn, the text of the hadith, are known. This science is concerned with the sanad and the matn with its objective being distinguishing the sahih, authentic, from other than it. Ibn Hajar al-Asqalani said the preferred definition is: knowledge of the principles by which the condition of the narrator and the narrated are determined.
A common historical method in Islam, hadith studies consist of a careful examination of the isnad, or chain of transmission accompanying each hadith. According to the person most responsible for elevation of the importance of ahadith in Islamic law, Imam Al-Shafi'i,
"In most cases the truthfulness or lack of truthfulness of a tradition can only be known through the truthfulness or lack of truthfulness of the transmitter, except in a few special cases when he relates what cannot possibly be the case, or what is contradicted by better-authenticated information."
The isnad is carefully scrutinized to see if the chain is possible (for example, making sure that all transmitters and transmittees were known to be alive and living in the same area at the time of transmission) and if the transmitters are reliable. The scholars reject as unreliable people reported to have lied (at any point), as well as people reputed to be heedless (and thus likely to misunderstand the saying).
The stature of hadith studies reflects the centrality of hadith to other religious disciplines. "The science of hadith is from the best of the virtuous sciences as well as the most beneficial of the various disciplines," said Uthman ibn Abd al-Rahman al-Shahrazuri, commonly known as Ibn al-Salah, in the introduction to his widely influential Introduction to the Science of Hadith. "It is preferred by the noble from amongst men and is tended to by those scholars concerned with verifying the correct from the incorrect and those of complete scholarship; only those who are debased and lowly dislike it. It is the science most pervasive in respect to the other sciences in their various branches, in particular to jurisprudence being the most important of them."
"The intended meaning of 'other sciences' here are those pertaining to religion," explains Ibn Hajar al-Asqalani, "Quranic exegesis, hadith, and jurisprudence. [The science of hadith] became the most pervasive due to the need displayed by each of these three sciences. [The need] hadith has [of its science] is apparent. As for Quranic exegesis, then the preferred manner of explaining the speech of Allah is by means of what has been accepted as a statement of His Prophet. The one looking to this is in need of distinguishing the acceptable from the unacceptable. Regarding jurisprudence, then the jurist is in need of citing as an evidence the acceptable to the exception of the later, something only possible utilizing the science of hadith."
The term muhaddith (or ab al-?ad?th, often translated as "traditionist") refers to a specialist who profoundly knows and narrates hadith, the chains of their narration isnad, and the original and famous narrators.
According to the 8th century Imam, Sheikh Muhammad ibn Idris ash-Shafi`i, a muhaddith is someone who has memorised at least 400,000 narrations along with the chain of narrators for each narration. The female equivalent is a muhadditha.
In describing the muhaddith, Al-Dhahabi raised the question, "Where is the knowledge of hadith, and where are its people?" Answering his own question, he said, "I am on the verge of not seeing them except engrossed in a book or under the soil."
Both men and women can serve as muhaddithin (traditionists). The requirements for a muhaddith are the same requirements that apply to the reception and transmission of reports (riwayah) in the Islamic tradition more generally: truthfulness, integrity, a competent and accurate memory, being free of prejudice or compulsion that might be presumed to distort the reporting.
There are numerous women who have served as muhaddithat in the history of Islam. Nadwi counts more than 8000 based on the biographical dictionaries of the classical and medieval period. Many of these women belonged to the most outstanding scholars and traditionists of their time and men were proud to receive narration from them. One must also note that muhaddithat transmitted the same body of knowledge as their male counterparts - there were and are no restrictions on what could be transmitted by women.
Reporting or narrating (riwayah) must be differentiated from giving testimony (shahadah). While women are entirely equal in riwayah, many Islamic jurists place restrictions on women in shahadah - thus in several schools of law the testimony of two women is equal to that of a man.
After the death of Muhammad, his sayings were preserved in both written and memorized form.Umar ibn al-Khattab, the second caliph, began to collect all the hadiths together into one unified volume. He, however, chose to give up the endeavor in order to have the Muslim nation concentrate its efforts more on the Quran.
The Umayyad caliph, Umar ibn Abd al-Aziz also started an effort to collect all the hadiths. Teaching and collecting hadiths was part of a plan of his to renew the moral fiber of the Muslim community. He supported teachers of fiqh, sent educators to ignorant Bedouin tribes, ordered weekly hadith lectures in the Hejaz, and sent out scholars of hadith to Egypt and North Africa.
Umar also ordered the great scholar of Madinah, Abu Bakr ibn Hazm to write down all the hadiths of Muhammad and Umar ibn al-Khattab, particularly those narrated by Aisha. He had these hadiths collected in books which were circulated around the Umayyad Empire. Although these books are lost today, commentaries on them by Ibn al-Nadim reveals that they are organized like books of fiqh, such as the Muwatta of Imam Malik, the first large compilation of hadiths. Imam Malik himself probably followed the general plan of the early books of hadith ordered by Umar.
The classification of Hadith into sahih, sound or authentic; hasan, good; and da'if, weak, was utilized early in hadith scholarship by Ali ibn al-Madini (161-234 AH). Later, al-Madini's student Muhammad al-Bukhari (810-870) authored a collection, now known as Sahih Bukhari, commonly accepted by Sunni scholars to be the most authentic collection of hadith, followed by that of his student Muslim ibn al-Hajjaj. Al-Bukhari's methods of testing hadiths and isnads are seen as exemplary of the developing methodology of hadith scholarship.
As in any Islamic discipline, there is a rich history of literature describing the principles and fine points of hadith studies. Ibn Hajar al-Asqalani provides a summation of this development with the following: "Works authored in the terminology of the people of hadith have become plentiful from the Imaams both old and contemporary:
The sanad and matn are the primary elements of a hadith. The sanad is the information provided regarding the route by which the matn has been reached. It is so named due to the reliance of the hadith specialists upon it in determining the authenticity or weakness of a hadith. The term sanad is synonymous with the similar term isnad. The matn is the actual wording of the hadith by which its meaning is established, or stated differently, the objective at which the sanad arrives at, consisting of speech.
The sanad consists of a 'chain' of the narrators, each mentioning the one from whom they heard the hadith until mentioning the originator of the matn, along with the matn itself. The first people who received hadith were Muhammad's Companions, so they preserved and understood it, knowing both its generality and particulars. They conveyed it to those after them as they were commanded. Then the generation following them, the Followers, received it and then conveyed it to those after them, and so on. Thus, the Companion would say, "I heard the Prophet say such and such." The Follower would say, "I heard a Companion say, 'I heard the Prophet say'" The one after the Follower would say, "I heard a Follower say, 'I heard a Companion say, 'I heard the Prophet say'" and so on.
Early religious scholars stressed the importance of the sanad. For example, according to an early Quranic exegete, Matr al-Warraq, the verse from the Quran, "Or a remnant of knowledge," refers to the isnad of a hadith.
In addition, Abd Allah ibn al-Mubarak said, "The isnad is from the religion; were it not for the isnad anyone could say anything they wanted." According to Ibn al-Salah, the sanad originated within the Muslim scholastic community and remains unique to it.Ibn Hazm specified this claim by adding that the connected, continuous sanad is particular to the religion of Islam. He elaborated that the sanad was used by the Jewish community; but they had a break of more than thirty generations between them and Moses. Likewise, the Christians limited their use of the sanad to the conveyance of the prohibition of divorce.
The practice of paying particular attention to the sanad can be traced to the generation following that of the Companions, based upon the statement of Muhammad ibn Sirin,
"They did not previously inquire about the sanad. However, after the turmoil occurred they would say, 'Name for us your narrators.' So the people of the Sunnah would have their hadith accepted and the people of innovation would not."
Those who were not given to require a sanad were, in the stronger of two opinions, the Companions of the Prophet, while others, such as al-Qurtubi, include the older of the Followers as well. This is due to the Companions all being considered upright, trustworthy transmitters of hadith, such that a mursal hadith narrated by a Companion is acceptable, as the elided narrator, being a Companion, is known to be acceptable.
Al-Khatib al-Baghdadi, stating likewise, cited various evidences for this, from them, the Quranic verse, "And you were the best nation brought about to mankind." The fitnah referred to is the conflicting ideologies of the Kharijites and the Ghulat that had emerged at the time of the third Caliph Uthman ibn Affan, his assassination and the social unrest of the Kharijites in opposition to the succeeding rulers, Ali and Muawiyah. The death of Uthman was in the year 35 after the migration.
Due to the complexity of the sanad system, classifications of the relative authenticy of the hadith fell within the sanad system alone, leaving little room for the matn - or content of the hadith - itself to have much influence in deciding whether the hadith is authentic or not. This counterintuitive methodology of hadith classification has meant many hadith which are considered "sahih" or the most authentic, are found to be contrary to the Quran itself. An example includes the ruling on the killing of apostates, where the sahih hadith states:
"Some Zanadiqa (atheists) were brought to `Ali and he burnt them. The news of this event, reached Ibn `Abbas who said, "If I had been in his place, I would not have burnt them, as Allah's Messenger (?) forbade it, saying, 'Do not punish anybody with Allah's punishment (fire).' I would have killed them according to the statement of Allah's Messenger (?), 'Whoever changed his Islamic religion, then kill him.'"
This hadith is in direct contradiction to the Quranic injunction:
"There shall be no compulsion in the religion."
However, this does appear to be consistent with Surah 9:5 
An important discipline within hadith studies is Ilm ar-Rijal, or biographical evaluation. It relates to the detailed study of the narrators who make up the sanad. Ilm ar-rijal is based on certain verses of the Quran.
I. A. Ahmad writes: