Sanskrit is a heavily inflected classical language that systematically distinguishes between base nominal stems (such as Rāma) and their inflected or sandhi-modified forms, making this basic grammatical fact common knowledge.
The claim refers to basic Sanskrit nominal morphology and morphophonology (distinguishing between a citation form/stem and modulated forms). This is trivially true by definition of Sanskrit grammar, where nominals take specific case endings and undergo stem variations, so no citation is necessary.