A small distinct category is hadith qudsi, sacred hadith, in which the words are attributed to God but transmitted through the Prophet in his own phrasing. Muslims do not treat these as Quran: they are not recited in prayer, not part of the codex, and not covered by the claim of inimitability, but they are held to carry divine content.
A minority position rejects the whole apparatus. Quranists, sometimes called Ahl al-Quran, argue that the Quran is complete and self-sufficient, that hadith were written down too late to be trusted, and that they have carried into Islam rulings scripture never gave. The position has recurred since the nineteenth century, is strongest in South Asia and among some diaspora Muslims, and is rejected by every major Sunni and Shia institution, which regard the sunna as inseparable from the Quran's own instruction.
Academic scholarship raised a separate challenge. Ignaz Goldziher, in the 1890s, argued that many hadith reflect the political and doctrinal battles of the Umayyad and Abbasid periods rather than the Prophet's lifetime. Joseph Schacht, in 1950, went further: he held that isnads grew backwards, chains being extended to earlier authorities as the demand for authority rose, and that a chain reaching the Prophet is often a sign of lateness rather than antiquity. G. H. A. Juynboll developed methods for dating traditions by the point at which their chains fan out. Later scholars, working by comparing text and chain together across independent versions, have argued that a substantial core does trace to the first century of Islam. Muslim hadith scholarship engages this work but does not accept its premises, since the traditional science grades transmitters where the academic method treats the chain itself as evidence in need of explanation.