The claim states a standard textbook fact in quantum field theory: local gauge invariance strictly forbids explicit (bare) mass terms for gauge fields (such as $m^2 A_ u A^ u$), forcing gauge bosons to acquire mass exclusively through mechanisms like spontaneous symmetry breaking (e.g., the Higgs mechanism). Because this is a direct, foundational definition and consequence of gauge symmetry taught universally in physics, it falls under common knowledge.