When ray of light passes through the center of curvature of the concave mirror it strikes along the normal that means it is incident on mirror at 90°.
Thus by the law of reflection which states that the angle of incidence is always equal to the angle of reflection i.e. 90°.
Hence the incident ray coincides with the normal and retraces its path after reflection from the spherical mirror.
The figure below illustrates the above situation.
