Śatapatha Brāhmaṇa ("of a hundred paths") 1.4.1.34

1.4.1.34

so 'nvāha | agniṃ dūtaṃ vṛṇīmaha iti devāśca vā asurāścobhaye prājāpatyāḥ paspṛdhire tāṃtspardhamānāṅgāyatryantarā tasthau yā vai sā gāyatryāsīdiyaṃ vai sā pṛthivīyaṃ haiva tadantarā tasthau ta ubhaya eva vidāṃ cakruryatarānvai na iyamupāvartsyati te bhaviṣyanti paretare bhaviṣyantīti tāmubhaya evopamantrayāṃ cakrire 'gnireva devānāṃ dūta āsa saharakṣā ityasurarakśasamasurāṇāṃ sāgnimevānupreyāya tasmādanvāhāgniṃ dūtaṃ vṛṇīmaha iti sa hi devānāṃ dūta āsīddhotāraṃ viśvavedasamiti

He recites [the eighth sâmidhenî]: 'Agni we choose as messenger!' Now the gods and the Asuras, both of them sprang from Pragâpati, were contending for superiority. When they were thus contending, the gâyatrî stood between them. That gâyatrî was the same as this earth, and this earth indeed lay between them. Now both of them knew that whichever she would side with, they would be victorious and the others would be defeated. Both parties then invited her secretly to come to them. Agni acted as messenger for the gods; and an Asura-Rakshas, named Saharakshas, for the Asuras. She then followed Agni: he therefore recites, 'Agni we choose for messenger,' because he was the messenger of the gods.--'As Hotri the all-knowing, him!'