Fermat’s principle of least time can be used to prove the following statement. Explain how?

Light always retraces its path.

In Fermat's Principle the optical path length between points A and B can be calculated in either direction, A to B or B to A, the result is the same. This leads to the principle of geometrical reversibility.

Principle of Ray Reversibility: Any actual ray of light in an optical system, if reversed in direction, will retrace the same path backward.