The increase in a black hole's mass through the accretion of matter and energy is a direct consequence of general relativity and black hole thermodynamics, making it a foundational and observational certainty that requires no citation.
The claim is a fundamental physical principle following directly from mass-energy equivalence and black hole mechanics (such as the increase in mass and area upon capturing matter/energy). It is trivially true by definition and observation in physics, placing it under common knowledge.