Śatapatha Brāhmaṇa ("of a hundred paths") 4.6.7.11

4.6.7.11

tadvā etadvṛṣā sāma | yoṣāmṛcaṃ sadasyadhyeti tasmānmithunādindro jātastejaso vai tattejo jātaṃ yadṛcaśca sāmnaścendra indra iti hyetamācakṣate ya eṣa tapati

Now there, in the Sadas, that male, the Sâman, longs after the female, the Rik. From that generation Indra was produced: from fire, indeed, fire is produced, viz. Indra from the Rik and the Sâman; for Indra they call him that burns yonder (the sun).