Śatapatha Brāhmaṇa ("of a hundred paths") 1.6.3.41

1.6.3.41

te vā ete | agnīṣomayoreva rūpamanvāyatte yacuklaṃ tadāgneyaṃ yatkṛṣṇaṃ tatsaumyaṃ yadi vetarathā yadeva kṛṣṇaṃ tadāgneyaṃ yacuklaṃ tatsaumyaṃ yadeva vīkṣate tadāgneyaṃ rūpaṃ śuṣke iva hi vīkṣamāṇasyākṣiṇī bhavataḥ śuṣkamiva hyāgneyaṃyadeva svapiti tatsaumyaṃ rūpamārdre iva hi suṣupuṣo 'kṣiṇī bhavata ārdra iva hi soma ājarasaṃ ha vā asmiṃ loke cakṣuṣmānbhavati sacakṣuramuṣmiṃloke sambhavati ya evametau cakṣuṣī ājyabhāgau veda

Those two (qualities), truly, are related to the natures of Agni and Soma: that which is white is related to Agni, and that which is black is related to Soma. If, however (it were asserted), on the contrary, that what is black is related to Agni, and what is white is related to Soma,--[the answer would be:--] what sees is of the nature of Agni, for dry, as it were, are the eyes of one who looks, and that which is dry relates to Agni;--and what sleeps is of the nature of Soma, for moist, as it were, are the eyes of one who is asleep, and moist also is Soma. And, verily, he who thus knows those two butter-portions to be eyes, remains endowed with eye-sight till old age in this world, and starts in yonder world possessed of eye-sight.