Rocket engine chamber pressure is determined by propellant mass flow rate and nozzle throat area as a matter of standard rocket propulsion physics, so no citation is needed.
The claim states a fundamental relationship in rocket propulsion (often expressed via the characteristic velocity and mass conservation equations where chamber pressure is proportional to mass flow rate times characteristic velocity divided by throat area). This is a textbook physical relationship taught in introductory aerospace engineering and constitutes common knowledge in the field, so no specific citation is required.