Background: Joseph Franz Schacht (born in Ratibor, 15 March 1902, died in Englewood, 1 August 1969) was a British-German professor of Arabic and Islam at Columbia University in New York. He was the leading Western scholar on Islamic law, whose Origins of Muhammadan Jurisprudence (1950) is still considered a centrally important work on the subject. The author of many articles in the first and second editions of the Encyclopaedia of Islam, Schacht also edited the second edition of The Legacy of Islam for the Legacy series of Oxford University Press and authored a textbook under the title An Introduction to Islamic Law (1964).
Context: Schacht locates the origins of 'ilm al-hadith in the eight and ninth centuries CE, a moment in the development of Islamic legal reasoning coinciding with the professionalization of the traditionalist (muhaddith) and jurist (faqih) classes in the urban centers of the Middle East. According to Schacht the rules and standards of Islamic law are derived from four primary sources: from the Qur`an, from the hadith comprising the sunna of the Prophet Muhammad and his Companions, from the consensus or ijma' of the Muslim community and from analogical reasoning or qiyas. The essentials of this fourfold methodology were initially and perhaps most clearly articulated by Imam al-Shafi`i, a noted legal scholar of the eighth and ninth centuries CE and founder of an eponymous school of Islamic jurisprudence. Following the work of Goldziher before him, Schacht argues that it was al-Shafi`i who first elevated the sunna and its constituent traditions to great legal prominence. The material importance of the Qur`an and hadith thereafter enjoyed a status comparable to that of juristic consensus, though for al-Shafi`i 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. Moreover, an already-existing legal standard based on such a tradition could be overturned only upon the emergence of a hadith that could be more credibly attributable to the Prophet. Al-Shafi`i goes as far as to claim that such well-established traditions invite no debate as to their validity; their truth simply imposes itself upon the human mind, leaving no room for doubt or speculation. Schacht points out, however, that al-Shafi`i inconsistently applies this rule in his own work, alleging that in some cases the jurist favored 'ahadith transmitted from Companions that openly contradicted those attributable to the Prophet. These traditions usually included those which validated ritual practices that were either universally agreed upon or else independently verifiable (such as daily prayer), and those which disputed the legal positions of al-Shafi`i's opponents. Building on this lattermost point, Schacht contends that far from constructing the standards of a legitimate epistemic enterprise al-Shafi`i's science of hadith amounts to little more than an uncritical acceptance of Prophetic traditions which justified his own legal preferences. These same traditions, claims Schacht, could not survive a stronger program of investigation. 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 'isnad criticism first articulated here by al-Shafi`i. Later in Origins Schacht presents evidence which in his estimation suggests that there was in fact a large scale fabrication of Prophetic 'isnads in the generation preceding the life of al-Shafi`i's own teacher, Malik ibn 'Anas (d. 795 CE). Even in Malik'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. For these reasons modern scholars cannot be nearly as optimistic about the historical-analytic value of hadith literature as were their medieval Muslim counterparts. CANNOTANSWER
Question: What other sources did he say Islamic law comes from?. Whats the answer?

from the hadith comprising the sunna of the Prophet Muhammad and his Companions, from the consensus or ijma' of the Muslim community and from analogical reasoning or qiyas.