That flow through a convergent-divergent nozzle throat reaches sonic velocity under chocked conditions is a foundational principle of compressible gas dynamics, meaning no citation is required.
The claim that the velocity through a rocket nozzle throat must be sonic is a fundamental, textbook definition of choked flow in compressible fluid dynamics (specifically for a de Laval nozzle). It represents standard engineering and physical first principles. Therefore, it is categorized as common knowledge and does not require empirical paper citations.