In the golden city of the heart dwells The Lord of Love, without parts, without stain. Know him as the radiant light of lights. There shines not the sun, neither moon nor star, Nor flash of lightning, nor fire lit on earth. The Lord is the light reflected by all. He shining, everything shines after him. Mundaka Upanishad 2.2.10-11
For more background about The Mundaka Upanishad: (Sanskrit: मुण्डक उपनिषद्, Muṇḍaka Upaniṣad) or the Mundakopanishad (Sanskrit: मुण्डकोपनिषद्, Muṇḍakopaniṣad)
commented upon by Shankara. It is associated with the Atharvaveda.
It figures as number 5 in the Muktika canon of 108 Upanishads.