This is a standard microeconomic definition regarding ordinal utility theory where indifference curves represent identical preference orderings; no citation is needed.
The claim states a fundamental definition in consumer choice theory: if two utility functions generate the same marginal rate of substitution everywhere (i.e., identical indifference curves), they represent the exact same ordinal preferences (related by a monotonic transformation). This is a textbook mathematical property and foundational knowledge in economics, rendering external empirical citations unnecessary.