Chandogya Upanishad 3.2

The southern rays of the sun are the honey-cells on the right. The Yagus verses are the bees, the Yagur-veda sacrifice is the flower, the water (of the sacrificial libations) is the nectar (of the flower).

Those very Yagus verses (as bees) brooded over the Yagur-veda sacrifice (the flower); and from it, thus brooded on, sprang as its (nectar) essence, fame, glory of countenance, vigour, strength, and health.

That flowed forth and went towards the sun. And that forms what we call the white (sukla) light of the sun.