This is a basic geometric and physical fact about planetary energy balance (dividing a disk interception area of pi times radius squared by a spherical surface area of four pi times radius squared gives one-fourth), so no formal citation is necessary.
The claim represents a fundamental, standard geometric calculation used in planetary energy balance models to convert solar constant (total solar irradiance) into an average surface insolation. It is a matter of basic mathematical definition and geometric derivation rather than an empirical hypothesis needing literature validation. Following the instructions for truisms and direct definitions, this is classified as COMMON_KNOWLEDGE.