The statement that the normal force is always perpendicular to the surface of contact is a fundamental definition in classical mechanics, meaning no citation is needed.
The claim defines 'normal force' (from 'norm', meaning perpendicular) as the component of a contact force that is perpendicular to the common boundary of the two interacting objects. This is a basic definition by everyday observation and introductory physics, making it common knowledge rather than an empirical hypothesis requiring literature verification.