Brihadaranyaka Upanishad 2.1.4

2.1.4

sahovāca gārgyo ya evāsau vidyuti puruṣa etamevāhaṃ brahmopāsa iti sa hovācājātaśatrurmā maitasminsaṃvadiṣṭhāstejasvīti vā ahametamupāsa iti sa ya etamevamupāste tejasvī ha bhavati tejasvinī hāsya prajā bhavatiṃ

Gârgya said: 'The person that is in the lightning (and in the heart), that I adore as Brahman.' Agâtasatru said to him: 'No, no! Do not speak to me on this. I adore him verily as the luminous.' Whoso adores him thus, becomes luminous, and his offspring becomes luminous.