Śatapatha Brāhmaṇa ("of a hundred paths") 6.2.1.9

6.2.1.9

sa aikṣata | yadi vā idamitthameva sadātmānamabhisaṃskaris pacānīti tadagnināpacattadenadamṛtamakarodetadvai haviramṛ\ye martya kuṇapo 'napahatapāpmā bhaviṣyāmi hantaitadagninātam bhavati yadagninā pacanti tasmādagnineṣṭakāḥ pacantyamṛtā evaināstatkurvanti

He considered, 'Surely, if I fit this (matter) such as it is unto mine own self, I shall become a mortal carcase, not freed from evil: well then, I will bake it by means of the fire.' So saying, he baked it by means of the fire, and thereby made it immortal; for the sacrificial food which is baked by fire is indeed immortal (or, ambrosia). Hence they bake the bricks with fire: they thereby make them immortal.