The standard Schrödinger equation is fundamentally non-relativistic by definition, as it is based on a non-relativistic energy-momentum relation, making this a matter of common knowledge that requires no citation.
The claim is a definitional property of the standard Schrödinger equation in quantum mechanics. It is a textbook fact that the standard Schrödinger equation uses a non-relativistic energy relation ($E = p^2/2m$), whereas relativistic quantum mechanics requires equations like Klein-Gordon or Dirac. Therefore, this is classified as common knowledge.