Omar Khayyám (1048-1131) was a Persian mathematician, philosopher, physician, and poet. Bellairs is likely referencing poet Edward FitzGerald’s English translation of The Rubaiyat of Omar Khayyam, a translation of about a thousand poems attributed to Khayyám, specifically stanza XVII:
Think, in this batter'd CaravanseraiA caravanserai a roadside inn having a central courtyard where caravans can rest.
Whose Doorways are alternate Night and Day,
How Sultan after Sultan with his Pomp
Abode his Hour or two, and went his way.