The Great Umayyad Mosque of Damascus built between about 706 and 714/15 CE is the oldest surviving mosque that still preserves large parts of its original architecture and decoration. The origins of the mosaic tesserae have been the subject of debate for more than a thousand years. The earliest sources written two centuries after the construction of the edifice claim a Byzantine origin of both, the material as well as the craftsmen. Here we use the compositional analyses of nearly 1,000 glass tesserae to show that 65% of the samples (80% of the coloured tesserae) from the mosque have a consistent chemistry and, by inference, originate from a common geographical source. Comparison with chemical data of early Islamic glass groups conclusively identifies Egypt as the origin of these tesserae and demonstrates that they are coeval with the foundation of the mosque. Additionally, the compositional features of the gold leaf tesserae testify to the systematic recycling and reuse of older material. Our findings suggest that the manufacture and supply of glass tesserae for the Great Mosque was a direct commission from the highest echelons of government.