A hadith is a narration of the sayings or actions of the Prophet Muhammad or his companions. To assess the authenticity of a hadith, scholars use a methodology called hadith criticism, which involves examining the chain of narrators and the content of the hadith to determine its reliability.
One reason a hadith may be considered unreliable is if it contradicts a Quranic injunction. The Quran is considered the primary source of Islamic teachings and beliefs, and any hadith that contradicts it is likely to be considered inauthentic.
Another reason a hadith may be considered unreliable is if its narrator was not considered trustworthy or reliable. Hadith scholars have a detailed system for evaluating the reliability of narrators, based on their character, memory, and other factors.
In addition, a hadith may be considered unreliable if its wording is too close to the text of the Quran or if it is no longer than a verse of the Quran. This is because the Quran is considered the ultimate authority in Islam, and any hadith that appears to mimic its language or content may be viewed with suspicion.
In summary, to determine the authenticity of a hadith, scholars examine the chain of narrators and the content of the narration. If a hadith contradicts the Quran, is narrated by an untrustworthy narrator, or appears to mimic the language or content of the Quran, it may be considered unreliable.