Prashna Upanishad - Chapter 1

He said: Prajapati created prana and rayi (matter). Of these two, prana is the Sun.

The Sun is indeed prana; the Moon is indeed rayi. Whatever is formed and formless — all that is rayi alone.

Sukesa Bharadvaja, Saibya Satyakama, Sauryayani Gargya, Kausalya Ashvalayana, Bhargava Vaidarbhi, and Kabandhi Katyayana — these, devoted to Brahman and established in Brahman, seeking the supreme Brahman, approached Pippalada.

The sage said to them: 'Live here for a year with austerity, celibacy, and faith. Then ask questions as you wish. If I know, I will tell you everything.'

Then Kabandhi Katyayana approached and asked: 'O Blessed One, from where are all these beings born?'

He said: 'Prajapati, desiring offspring, performed austerity. Having performed austerity, he produced a pair — rayi and prana — thinking: these two will produce beings for me in many ways.'

The Sun is indeed prana; the Moon is indeed rayi. Whatever is formed and formless — all that is rayi. Therefore form itself is rayi.

That which is prana is indeed the year. Its two paths are the southern and the northern.

As all these creatures go day by day without finding death — but when they depart, they go to death.