More poetic and romantic:
* Your eyes are a mirror to the beauty of your soul. (Classic and elegant)
* I see the beauty of your soul reflected in your eyes. (More direct and personal)
* Your eyes hold the beauty of your soul within them. (Emphasizes the eyes as containers of beauty)
* In your eyes, I see the radiance of your soul. (Highlights the shining quality of the soul's beauty)
More straightforward and sincere:
* Your eyes reflect the kindness (or goodness, light, purity) of your soul. (More specific about the type of beauty)
* Looking into your eyes, I see the beauty of your soul. (Emphasizes the act of looking)
More figurative and artistic:
* Your eyes are windows to the beauty within. (Classic and widely understood)
* Your eyes paint a portrait of your beautiful soul. (More creative and descriptive)
The best option for you will depend on the context of what you want to use the line.
Consider the context:
* Who are you talking to? A loved one? A friend? A stranger?
* What is the situation? Are you trying to be romantic? Express admiration? Offer comfort?
Good luck choosing the perfect phrase!