See Dale Eickelman and James Piscatori, “Social Theory in the Study of Muslim Societies,” in Muslim Travellers: Pilgrimage, Migration and the Religious Imagination. D. Eickelman and J. Piscatori, eds. (Berkeley: University of California Press, 1990), 3–28; Muhammad K. Masud, “The Obligation to Emigrate: The Doctrine of Hijra in Islamic Law” in Muslim Travellers, 29–49; Roger Webster. “Hijra and Dissemination of Wahhabi Doctrine in Saudi Arabia,” in Golden Roads: Migration. Pilgrimage and Travel in Mediaeval and Modern Islam. Ian Netton, ed. (Surrey, UK: Curzon Press, 1993), 11–27.