The Sefer Asaf, the earliest known Hebrew work on medicine, is "extant in 16 manuscripts, some complete; it constitutes a source of information on ancient customs and Jewish medical ethics as well as of ancient Jewish remedies and Hebrew, Aramaic, Persian, Latin, and Greek medical terminology. Excerpts from Greek medical books, some of which have been lost and are not known from any other sources, appear in Hebrew in this book. The most complete manuscripts are in Munich, Oxford, Brit. Museum London, Florence, and Paris. The book was not written by Asaph himself, but by his disciples. They mention, as teachers, R. Johanan b. Zavda and R. Judah ha-Yarhoni, as well as Asaph. Some sections of the book are very old, though others were written or translated from other languages as late as the seventh until the tenth century. The antiquity of the work is apparent from its style, similar to that of the older Midrashim, from its use of Persian (rather than Arabic) synonyms, and from the mention of weights current in Palestine during the talmudic period" (Encyclopedia Judaica, 2008)

See also, Muntner, "The antiquity of Asaph the Physician and his editorshoip of the earliest Hebrew book of medicine," Bull. Hist. Med., 25 (1951) 101-131.