The common mathematical model for cyber-physical systems is that of hybrid systems that enable combining both discrete and continuous behaviors represented by differential equations. In this paper, we introduce a formal approach, using EVENT-B and its refinement strategy, for specifying and verifying cyber-physical systems whose behavior is described by ordinary differential equations. To deal with the resolution of ordinary differential equations in EVENT-B, the approach is based on interfacing the differential equation solver SAGEMATH (System for Algebra and Geometry Experimentation) with the RODIN tool, a platform for EVENT-B projects development. For this purpose, we modeled and implemented the interface to the solver in EVENT-B using a RODIN plugin. This enables to reason on the EVENT-B specification and prove safety properties. The proposed approach was successfully applied on a frequently used cyber-physical system case studies.