Śatapatha Brāhmaṇa ("of a hundred paths") 2.2.2.8

2.2.2.8

devāśca vā asurāśca | ubhaye prājāpatyāḥ paspṛdhire ta ubhaya evānātmāna āsurmartyā hyāsuranātmā hi martyasteṣūbhayeṣu martyeṣvagnirevāmṛta āsa taṃ ha smobhaye 'mṛtamupajīvanti sa yaṃ ha smaiṣāṃ ghnanti taddha sma vai sa bhavati

Now the gods and the Asuras, both of them sprung from Pragâpati, were contending with each other. They were both soulless, for they were mortal, and he who is mortal is soulless. Among these two (classes of beings) who were mortal, Agni alone was immortal; and it was through him, the immortal, that they both lived. Now whichsoever (of the gods) they (the Asuras) slew, he, indeed, was so (slain).