Muhaddith (Arabic: محدث) is an Islamic scholar who studies ahadith (sayings of the Prophet) and the science and methods of studying ahadith.
Muhaddiths collected the sayings of the Prophet Muhammad (called ahadith). They checked if the chain of people who told the hadith (called narrators, or rawis) was trustworthy. They also tried to understand how each report reached them.
When they studied all the details about ahadith, muhaddiths also became narrators themselves. Every muhaddith was also a rawi (narrator), but not every rawi was a muhaddith.
A similar word to muhaddith is “musnid”. Musnids are narrators of ahadith who mention the chains of narrators (rawis), but they may not know extra details about them. Muhaddiths knew these extra details, so their level was higher than musnids.
A muhaddith who memorizes more than one hundred thousand hadith texts (matn), their chains of narrators (isnad), and also knows the personal qualities of the narrators is called a “guardian” (hafiz). If a hafiz knows more than three hundred thousand ahadith, they are called a “proof” (hujjah).
The writers of hadith books called “Sunan” are called ashab as-Sunan. These include:
The first four are known as the most important authors of the “Sunan” books and are called ashab as-Sunan al-arba'a.