Karamustafa, Ahmet T., Sufism: the Formative Period, Edinburgh, Edinburgh University Press, 2007, pp. 90-91. C. Melchert has covered this area of research in several publications. See: “The Ḥanābila and the Early Sufis”, Arabica, Vol. 48, No. 3, 2001, pp. 352-367; “Early Renunciants as Ḥadīth Transmitters”, The Muslim World, Vol. 92, No. 3-4, 2002, pp. 407-418; “The Piety of the Ḥadīth Folk”, International Journal of Middle East Studies, Vol. 34, No. 3, 2002, pp. 425-439.
Silvers, Laury, “Theoretical Sufism in the Early Period: With an Introduction to the Thought of Abū Bakr al-Wāsiṭī (d. ca. 320/928) on the Interrelationship between the Theoretical and the Practical Sufism”, Studia Islamica, No. 98/99, 2004, p. 72.
Rosen, Lawrence, Law as Culture: An Invitation, Princeton, Princeton University Press, 2006. pp. 99-100. However, I do not agree with Rosen that an Islamic court is based exclusively on persons in opposition to Western courts based on facts. The jurists also emphasize investigation that uncovers facts as has been demonstrated by Ibn Qayyim al-Jawziyya in his book al-Ṭuruq al-ḥukmiyya fī ’l-siyāsa al-sharʿiyya.