Redox half-reactions describe the conceptual or physical separation of oxidation and reduction processes, which forms the foundational basis of electrochemistry; no citation is needed.
The claim that redox half-reactions correspond to physically separable oxidation and reduction processes is a foundational, definitional concept of electrochemistry (e.g., in galvanic and electrolytic cells, half-cells are physically separated by salt bridges or membranes). Under the prompt instructions, trivially true definitions or direct chemical observations of this nature are classified as common knowledge.