The expression for the classical partition function of an ideal gas involving Planck's constant and the factorial of the particle number is a foundational textbook formula in statistical mechanics, representing standard background knowledge that does not require citation.
The claim states the standard expression for the classical partition function of $N$ identical particles ($Z = \frac{1}{N! h^{3N}} \int \dots$). This is a definition and a fundamental result taught in standard physics coursework, making it common knowledge not requiring specific paper citations.