TL;DR: FitzGerald made it up. Why? We don't know.
As I expected, the translations of Rubaiyat are all over the place when it comes to sticking to the original text. First, I came across an analysis or FitzGerald's translation, which says the following:
The sense here is presumably one of a disembodied Voice calling from the Great Unknown.
But, this ...
It can depend on how you read the word "collected", and also how genuine you believe FitzGerald's version - with its omissions and insertions - to be.
11th century - 12th century
The Rubaiyat had been popular in Persia since the 11th century. They were occasional verses, but there is nothing found to show they were brought together and published as a ...