Al-Muḥaddith (Arabic: المُحَدِّث), or a scholar of hadiths, is an expert of hadiths, transmitters of hadiths, terminologies of hadiths, reliable and unreliable hadiths, and problems with hadiths. Most muhaddiths wrote large hadith books.
The best-known Shiite muhaddiths include al-Kulayni, al-Shaykh al-Saduq, and al-Shaykh al-Tusi, the authors of the Four Books. Al-'Allama al-Majlisi and al-Shaykh al-Hurr al-'Amili were well-known muhaddiths in the Safavid period. In later periods, Mirza Husayn al-Nuri, the author of Mustadrak al-wasa'il and Shaykh 'Abbas Qummi, the author of Safinat al-bihar, came to be known as muhaddiths.
According to al-Mamaqani in his book, Miqbas al-hidaya, muhaddith is a person who knows how to establish the reliability of a hadith, the names of its transmitters, and their righteousness, as well as whether or not something is added to, or eliminated from a hadith. In his commentaries on Miqbas al-hidaya, Muhammad Rida Mamaqani takes a muhaddith to be a person who knows the science of hadith and its terminologies as well as the terms of abstruse hadiths (Gharib al-hadith) and types of hadiths. He quotes Kashshaf istilahat al-funun as suggesting that a muhaddith is characterized by hearing, writing, reciting, and memorizing hadiths, as well as traveling to different places in order to hear hadiths. According to al-Shahid al-Thani, a muhaddith is a person who is engaged with the tradition of the Prophet (s) and Imams (a) in a scholarly way.
According to al-Mamaqani, if a person is referred to as a muhaddith in a source of rijal, that would count as a praise, although it does not amount to that person's reliability.
Difference between Muhaddith, Transmitter of Hadiths, and Scholar of Rijal
Al-Mamaqani has compared terms such as "rawi" (transmitter of hadiths), "musnid" (transmitter of hadiths through chains of transmission), "muhaddith", and "hafiz" (memorizer of hadiths) with one another. According to him, a transmitter of hadiths is a person who just transmits a hadith, whether or not he mentions the chain of transmission. the transmitter of hadiths is of a lower ranking than a muhaddith. He makes it explicit that a person is not called a "muhaddith" just by having heard a hadith.