In standard macroeconomic theory, the LM curve slopes upward because a higher level of income increases money demand, which requires a higher interest rate to maintain equilibrium in the money market.
The claim is a fundamental definition from standard macroeconomics (the IS-LM model) taught in every introductory and intermediate economics textbook. None of the retrieved papers discuss macroeconomics or the IS-LM model; they pertain to topics such as medical risk scoring, environmental pollution, physics models, and pest assessments. As a textbook definitional fact, no citation is needed.