This is a fundamental thermodynamic definition stating that heat capacity is the derivative of internal energy with respect to temperature using natural variables, which is universally accepted textbook knowledge.
The claim states a foundational thermodynamic relation (specifically, isochoric heat capacity as the partial derivative of internal energy with respect to temperature at constant volume and entropy/natural variables). Because this is a definitional identity taught in introductory thermodynamics, it is classified as common knowledge requiring no citation.