The Rubaiyat of Omar Khayyám is a series of short poems (rubaiyat means "quatrains") written in the 11th and 12th centuries by Omar Khayyám, a Persian poet, mathematician and astronomer. They were translated into English in the latter half of the 19th century by the English writer Edward FitzGerald.
Although Khayyám is supposed to have written one or two thousand quatrains, FitzGerald's editions contain only about a hundred. And FitzGerald's translations are not exact. Wikipedia says: "Many of the verses are paraphrased, and some of them cannot be confidently traced to any one of Khayyam's quatrains at all."[1] Nevertheless, these remain the best-known English versions of Khayyám's work.
This web site includes FitzGerald's first and fifth editions, chiefly because that was what was available from Project Gutenberg.
1: See https://en.wikipedia.org/wiki/Rubaiyat_of_Omar_Khayyam#Edward_FitzGerald_versions, or, for the specific revision, https://en.wikipedia.org/w/index.php?title=Rubaiyat_of_Omar_Khayyam&oldid=755582253#Edward_FitzGerald_versions.