Through the courtesy of the authorities of the Preussische Staatsbibliothek I have had the opportunity of reading in the India Office Library the apparently unique copy of Abū Sa'd 'Abd al-Malik b. Muhammad b. Ibrāhīm al-Wā'iz al-Khargūshī's manual of Sūfism entitled Tahdhīb al-asrār. Khargūshī, who is otherwise known for his voluminous biography of the Prophet and a treatise on the interpretation of dreams, died at Nīshāpūr in 406/1015 or 4Q7/1016. He is therefore an earlier authority than Sulamī (d. 412/1021), Abu Nu'aim al-Isfahānī (d. 430/1039), and Qushairī (d. 465/1072), but is junior to Sarrāj (d. 378/988), Kalābādhī (d. 385/995), and Abū Tālib al-Makkī (d. 386/996). These comparisons are important, for they show Khargūshī as being a sufficiently early writer in the genre of systematic Sūfism. His name is not included in Nicholson's list of eight authorities for the history of early Sufism, and he does not appear to have received the attention which he deserves; though it is true that he has been utilized by Massignon in his study of Hallaj. The purpose of the present note is to supplement the somewhat exiguous account of the work given by Ahlwardt, and also to estimate the true value of Khargūshī's manual as a primary source.