The claim that an isothermal curve lies above an adiabatic curve during compression follows directly from the adiabatic index (gamma > 1) in ideal gas thermodynamics and basic P-V diagram geometry; no citation is needed.
The claim describes a fundamental, textbook thermodynamic property of gases (since PV^gamma = constant for adiabatic processes and PV = constant for isothermal processes, with gamma > 1, the adiabatic curve is steeper than the isothermal one, meaning during compression the isothermal curve resides above the adiabatic curve in pressure-volume coordinates). This is trivially true by definition and everyday observation in thermodynamics, making it common knowledge requiring no citation to specific research papers.