Śatapatha Brāhmaṇa ("of a hundred paths") 2.2.4.1

2.2.4.1

prajāpatirha vā idamagra eka evāsa | sa aikṣata kathaṃ nu prajāyeyeti so 'śrāmyatsa tapo 'tapyata so 'gnimeva mukhājjanayāṃ cakre tadyadenam mukhādajanayata tasmādannādo 'gniḥ sa yo haivametamagnimannādaṃ vedānnādo haiva bhavati

Pragâpati alone, indeed, existed here in the beginning. He considered, 'How may I be reproduced?' He toiled and performed acts of penance. He generated Agni from his mouth; and because he generated him from his mouth, therefore Agni is a consumer of food: and, verily, he who thus knows Agni to be a consumer of food, becomes himself a consumer of food.