Śatapatha Brāhmaṇa ("of a hundred paths") 10.2.5.2

10.2.5.2

yadvevāntaropasadau cinoti etadvai devā abibhayuryadvai na imamiha rakṣāṃsi nāṣṭrā na hanyuriti ta etānvajrānapaśyannupasado vajrā vā upasadastānprāpadyanta tānprapadyābhaye'nāṣṭra etamātmānaṃ samaskurvata tathaivaitadyajamāna etānvajrānprapadyābhaye'nāṣṭra etamātmānaṃ saṃskurute

And, again, as to why he builds between the Upasads. At this time the gods were afraid lest the fiends, the Rakshas, should destroy that (Agni's body) of theirs (built) there. They saw these thunderbolts, the Upasads, for the Upasads indeed are thunderbolts: they entered them, and, having entered them, they completed that body in a place free from danger and devilry; and in like manner does the Sacrificer now, after entering those thunderbolts, complete this body (of Agni) in a place free from danger and devilry.