The statement that natural spontaneous processes proceed in the direction of an increase in total entropy is a fundamental formulation of the second law of thermodynamics and represents a foundational principle of physics that requires no specific citation.
The claim expresses the second law of thermodynamics, which is a well-established, universally accepted fundamental principle of physics and chemistry. Per the instructions, statements that are trivially true by definition or direct foundational consensus are classified as common knowledge, requiring no citation or paper assignment.