He was the leading Western scholar in the areas of Islamic law and hadith studies, whose Origins of Muhammadan Jurisprudence (1950) is still considered a centrally important work on the subject.
In 1924 he published his Habilitations-Schrift, Das kitab al-hiial fil-fiqh (Buch d. Rechtskniffe) des abū Hātim Mahmūd ibn al-Hasan al-Qazuīnī, with translation and commentary.
[9] Schacht argues that in part the fabrication of ahadith came from "a literary convention, which found particular favor in Iraq", whereby authors/scholars would put their "own doctrine or work under the aegis of an ancient authority.
[14] As evidence that most ahadith were created after 100 A.H., Schacht notes that: As a whole, Origins critiques the methods and standards of ḥadīth verification as they were first articulated by Al-Shafi‘i and subsequently developed by his students in the eighth and ninth centuries CE, an early and centrally important stage in the formation of Islamic jurisprudence.
[20] Schacht locates the origins of 'ilm al-ḥadīth in the eighth and ninth centuries CE, a moment in the development of Islamic legal reasoning coinciding with the professionalization of the traditionalist (muḥaddith) and jurist (faqīh) classes in the urban centers of the Middle East.
The material importance of the Qurʾān and ḥadīth thereafter enjoyed a status comparable to that of juristic consensus, though for al-Shāfiʻī traditions credibly attributed to the Prophet were to be considered more authoritative than those of his Companions, and indeed could supersede all other sources of legal authority.
[31][32] Schacht points out, however, that al-Shāfiʻī inconsistently applies this rule in his own work, alleging that in some cases the jurist favored 'aḥādīth transmitted from Companions that openly contradicted those attributable to the Prophet.
[33][34] Building on this lattermost point, Schacht contends that far from constructing the standards of a legitimate epistemic enterprise al-Shāfiʻī's science of ḥadīth amounts to little more than an uncritical acceptance of Prophetic traditions which justified his own legal preferences.
[35] Although the technical evaluation of traditions would continue to evolve across many generations of Muslim scholars, it seems to have largely proceeded along the lines of the deficient form of ’isnād criticism first articulated here by al-Shāfiʻī.
[36] Later in Origins Schacht presents evidence which in his estimation suggests that there was in fact a large scale fabrication of Prophetic ’isnāds in the generation preceding the life of al-Shāfiʻī's own teacher, Mālik ibn ’Anas (d. 795 CE).
Even in Mālik's esteemed golden narrative chain there are suspicious gaps and obvious substitutions, sowing significant doubts as to credibility of the relationships he was said to have had with certain key transmitters.
According to Hallaq, it is more likely the case — and indeed apparent to the careful reader upon inspecting the literature — that at least where matters of law were concerned medieval Muslim scholars judged the majority of ḥadīth as only probabilistically true.