Lucretius talks about the motion of dust in light, but he doesn't claim that it is the origin of the theory. When I google "Leucippus dust light" I get lots of people making my claim and more respectable sources making weaker claims, like "According to traditional accounts the philosophical idea of simulacra is linked to Leucippus’ contemplation of a ray of light that made visible airborne dust," but I don't see any citations to where this tradition is recorded.