Śatapatha Brāhmaṇa ("of a hundred paths") 7.3.1.26

7.3.1.26

tadāhuḥ | kathamasyaitāḥ pakvāḥ śṛtā upahitā bhavantīti raso vā etāḥ svayaṃśṛta u vai raso 'tho yadvai kiṃ caitamagniṃ vaiśvānaramupanigacati tata eva tatpakvaṃ śṛtamupahitam bhavati

As to this they say, 'How do these (clod-bricks) come to be put on as baked, as heated (burnt) ones?' Well, these (clods) are vital sap, and the vital sap (blood) is naturally-heated; and, moreover, whatever comes in contact with Agni Vaisvânara, even thereby comes to be put on as baked, as heated.