While individual radioactive decay is a fundamentally random quantum process, large collections of radioactive atoms obey statistical laws such as exponential decay and half-life, yielding predictable macroscopic behavior. No citation is needed for this basic principle of statistics and physics.
The claim describes a fundamental statistical property of radioactive decay (the law of large numbers applied to quantum events), which is a matter of everyday physical and mathematical observation. Therefore, it is categorized as common knowledge and does not require citations from the retrieved literature.