^Esposito, John (2004), The Oxford Dictionary of Islam, Oxford paperback reference, Oxford, UK: Oxford University Press, p. 22, ISBN0-19-512559-2
^Moezzi, Mohammad Ali Amir (1994), The Divine Guide in Early Shiʻism: The Sources of Esotericism in Islam, Albany: State University of New York Press, p. 6, ISBN0-7914-2121-X