The distinction between special relativity (flat Minkowski spacetime) and general relativity (curved spacetime) is a foundational definition in physics and requires no citation.
The claim states a foundational, definitional fact about the two core theories of relativity: special relativity is formulated on flat (Minkowski) spacetime, whereas general relativity describes gravity in terms of dynamical spacetime curvature. This is common knowledge in physics, so no citations are necessary.