Jaiminīya Upaniṣad Brāhmaṇa 3.34.6

3.34.6

tad yathā ha vai suvarṇaṃ hiraṇyam agnau prāsyamānaṃ kalyāṇataraṃ kalyāṇataram bhavaty evam eva kalyāṇatareṇa kalyāṇatareṇātmanā sambhavati ya evaṃ veda /

Now, just as golden gold thrown into the fire becomes ever fairer and fairer, even so he comes into being with an ever fairer and fairer self, who knows thus.