The statement that the acceleration of an object in Earth's gravity is independent of its mass (the universality of free fall, arising from the equivalence of inertial and gravitational mass) is a foundational and thoroughly verified principle of physics, making further citation unnecessary.
The claim states a fundamental law of classical mechanics and general relativity (Galileo's principle of equivalence / weak equivalence principle). As a basic everyday physical observation and defining property of gravity in standard physics, it qualifies as common knowledge and does not require a citation, though several retrieved papers ([0], [6]) touch upon its advanced experimental verification.