The internal energy of an ideal gas depends solely on its temperature and is independent of its volume (Joule's second law), making this statement a textbook definition that does not require citation.
The claim states a fundamental law of thermodynamics regarding ideal gases (often referred to as Joule's law or a direct consequence of the ideal gas equation and internal energy dependence solely on temperature). As a foundational definition taught in introductory physics and chemistry, it is common knowledge in thermodynamics and does not need empirical citation.