The authenticity of the poetry attributed to Omar Khayyam is highly uncertain. Omar was famous during his lifetime not as a poet but as an astronomer and mathematician. The earliest reference to his having written poetry is found in his biography by al-Isfahani, written 43 years after his death. This view is reinforced by other medieval historians such as Shahrazuri (1201) and Al-Qifti (1255). Parts of the Rubaiyat appear as incidental quotations from Omar in early works of biography and in anthologies. These include works of Razi (ca. 1160–1210), Daya (1230), Juvayni (ca. 1226–1283), and Jajarmi (1340).:92:434 Also, five quatrains assigned to Khayyam in somewhat later sources appear in Zahiri Samarqandi's Sindbad-Nameh (before 1160) without attribution.:34
The number of quatrains attributed to him in more recent collections varies from about 1,200 (according to Saeed Nafisi) to more than 2,000. Skeptical scholars point out that the entire tradition may be pseudepigraphic.:11 The extant manuscripts containing collections attributed to Omar are dated much too late to enable a reconstruction of a body of authentic verses.
In the 1930s, Iranian scholars, notably Mohammad-Ali Foroughi, attempted to reconstruct a core of authentic verses from scattered quotes by authors of the 13th and 14th centuries, ignoring the younger manuscript tradition. After World War II, reconstruction efforts were significantly delayed by two clever forgeries. De Blois (2004) is pessimistic, suggesting that contemporary scholarship has not advanced beyond the situation of the 1930s, when Hans Heinrich Schaeder commented that the name of Omar Khayyam "is to be struck out from the history of Persian literature".
A feature of the more recent collections is the lack of linguistic homogeneity and continuity of ideas. Sadegh Hedayat commented that "if a man had lived for a hundred years and had changed his religion, philosophy, and beliefs twice a day, he could scarcely have given expression to such a range of ideas".:34 Hedayat's final verdict was that 14 quatrains could be attributed to Khayyam with certainty. Various tests have been employed to reduce the quatrains attributable to Omar to about 100.:434 Arthur Christensen states that "of more than 1,200 ruba'is known to be ascribed to Omar, only 121 could be regarded as reasonably authentic".:663 Foroughi accepts 178 quatrains as authentic, while Ali Dashti accepts 36 of them.:96
FitzGerald's source were transcripts sent to him in 1856–1857 by his friend and teacher Edward B. Cowell of two manuscripts, a Bodleian manuscript with 158 quatrains, and a "Calcutta manuscript".
FitzGerald completed his first draft in 1857 and sent it to Fraser's Magazine in January 1858. He made a revised draft in January 1859, of which he privately printed 250 copies. This first edition became extremely sought after by the 1890s, when "more than two million copies ha[d] been sold in two hundred editions".