Where does the idea that dragons hoard gold come from?
I can't think of many myths of dragons hoarding gold. The closest I come up with is the dragon that guarded the Golden Fleece. And even that's a stretch as it wasn't gold specifically.
The Colchian dragon isn't the only dragon protecting treasure in Greek mythology. A couple more examples:
However, if you are looking for a dragon protecting gold specifically, then I think the more characteristic examples from European folklore are:
In fact, unless I'm horribly mistaken, the latter one is often credited as the main inspiration behind Smaug, the iconic gold-hoarding dragon in J. R. R. Tolkien's The Hobbit.
The earliest appearance in Western history and literature is in Herodotus. The Scythians are famous for their gold art objects. He says that way up north in the Altai mountains where the Scythians mined their gold and made their gold objects, there were gryphons guarding piles of gold. The gryphon was a four footed bird. That mythical creature easily turned into the dragon.