This claim follows directly from the definition of unitary demand elasticity where the percentage change in quantity demanded exactly offsets the percentage change in price, leaving total revenue unchanged. No citation is needed for this definitional economic principle.
The claim states a foundational economic definition regarding price elasticity of demand equal to one (unitary elasticity), where total revenue ($P \times Q$) remains invariant to small price changes because the percentage change in quantity is equal and opposite to the percentage change in price. This is a matter of mathematical definition and basic economic observation rather than an empirical hypothesis requiring new literature validation.