Śatapatha Brāhmaṇa ("of a hundred paths") 2.2.4.2

2.2.4.2

tadvā enametadagre devānāmajanayata | tasmādagniragnirha vai nāmaitadyadagniriti sa jātaḥ pūrvaḥ peyāya yo vai pūrva etyagra etīte vai tamāhuḥ so evāsyāgnitā

He thus generated him first (agre) of the gods; and therefore (he is called) Agni, for agni (they say) is the same as agri. He, being generated, went forth as the first (pûrva); for of him who goes first, they say that he goes at the head (agre). Such, then, is the origin and nature of that Agni.