The vast amount of ahadith that exist today began when the sahabah and those who came after them began to memorise, write down, and pass on the statements of Prophet Muhammad pbuh and descriptions of his actions. As many outstanding individuals began to collect hundreds of thousands of narrations, it became necessary to find a way to distinguish those reports that were true from those that were false. The methodology that evolved became the science of hadith. It sorts genuine sayings from those that are not and classifying them into certain categories.
Hadith classification is a meticulous science and involves strict adherence to the guidelines built up over several centuries. There are many ways to classify ahadith and before you read a hadith it has gone through several methods of classification, including but not limited to, defects found in the matn or isnad, how many reporters are there in the isnad, or the manner in which the matn is reported.
However, the most well-known and most visible way is to classify ahadith according to the reliability and memory of the reporters. With this method, the ahadith are classified as follows. Each hadith is said to be saheeh (sound), hasan (good), daeef (weak) or mawdu (fabricated or forged).
These is the most authentic and reliable class of ahadith. According to hadith scholar Ibn As-Salah (1181 -1245 CE), a saheeh hadith is one that has a continuous isnad, made up of reporters with trustworthy memory and is found to be free from any irregularities in either the matn or the isnad.
The term hasan means good, and Ibn As-Salah describes a hasan hadith as one that is a degree less than a saheeh hadith. It is free from irregularities in both the matn and the isnad but one or more of the reporters may have a less reliable memory, or the hadith falls short of the strict rules of saheeh classification.
Both saheeh and hasan ahadith can be used to support or make a point of law. Several ahadith considered to be daeef can raise each other to the degree of hasan if the weakness in the reporters is considered to be mild. However, if the weakness is severe, the hadith will remain daeef.
A hadith that fails to reach the status of hasan is known as daeef. Usually, the weakness is a broken isnad or that one or more of the reporters have a character defect. He or she may be known to tell lies, make excessive mistakes, oppose the narrations of a more reliable source, be involved in innovation, or have some other ambiguity of character.
A mawdu hadith is one that is either fabricated or forged. The matn of a mawdu hadith usually goes against established norms or contains some error or discrepancy in the dates or times of a particular incident. There are however many reasons why ahadith were fabricated and these include, political animosity, fabrication by storytellers, proverbs turned into ahadith, personal prejudices and deliberate misleading propaganda.