That acceleration is absolute while absolute velocity does not exist is a foundational principle of both classical mechanics and relativity, representing standard physics that requires no citation.
The claim states a fundamental axiom of kinematics and relativity (namely, that velocity depends on the choice of inertial reference frame, whereas acceleration is frame-independent and can be measured locally without reference to an external coordinate system). This is a well-established, definitional fact of physics taught in introductory textbooks, making it common knowledge rather than a contestable research hypothesis.