Shatapatha Brahmana 8.1.1.4

8.1.1.4

sa purastādupadadhāti | ayam puro bhuva ityagnirvai purastadyattamāha pura iti prāñcaṃ hyagnimuddharanti prāñcamupacarantyatha yadbhuva ityāhāgnirvai bhuvo 'gnerhīdaṃ sarvam bhavati prāṇo hāgnirbhūtvā purastāttasthau tadeva tadrūpamupadadhāti

In front (of the altar) he lays down (ten bricks,--the first) with (Vâg. S. XIII, 54), 'This one in front, the Existent,'--in front, doubtless, is Agni; and as to why he speaks of him (as being) 'in front,' it is because they take out the fire (from the Gârhapatya) towards the front, and attend on Agni towards the front. And as to why he says 'the existent (bhuva),' Agni is indeed the existent, for it is through Agni that everything exists (bhû) here. Agni, indeed, having become the breath, remained in front: it is that very form he now bestows (on Agni).