Shatapatha Brahmana 1.6.1.11

1.6.1.11

te svargaṃ lokaṃ yantaḥ | asurarakṣasebhya āsaṅgādbibhayāṃ cakruste 'gnim purastādakurvata rakṣohaṇaṃ rakṣasāmapahantāramagniṃ madhyato 'kurvata rakṣohaṇaṃ rakṣasāmapahantāramagnim paścādakurvata rakṣohaṇaṃ rakṣasāmapahantāraṃ

In going to heaven they were afraid of an attack from the Asuras and Rakshas. They placed Agni at their head, as the Rakshas-killer, the repeller of the Rakshas; they placed Agni in their midst, as the Rakshas-killer, the repeller of the Rakshas; they placed Agni in their rear, as the Rakshas-killer, the repeller of the Rakshas.