“The Sabians were allowed to build a new Temple of the Moon God, and to continue their religious rites, after the Arab General Ibn Ghanam conquered Harran in the seventh century AD. This in itself is a sign of most unusual favor, since Islamic armies normally offered "pagans" the choice of either conversion or death. Even more interesting, however, is the Sabians' encounter with the Abbasid Caliph Abu Jafar Abdullah al-Ma'mun, who passed through their city in AD 830 and reportedly quizzed them intensively on their religion.
Remembering the Sabian pilgrimages to Giza, it is reasonable to wonder whether there is any connection with the fact that in AD 820, a decade before he visited Harran, it was Ma'mun who tunnelled into the Great Pyramid and opened its previously hidden passageways and chambers. Indeed, it is through "Ma'mun's Hole" that visitors still enter the monument today. Described by Gibbon as "a prince of rare learning," it seems Ma'mun's investigation was prompted by information he'd received about the Great Pyramid, specifically that it contained: 'a secret chamber with maps and tables of the celestial and terrestrial spheres. Although they were said to have been made in the remote past, they were suppposed to be of great accuracy.”