Shatapatha Brahmana 10.2.5.1

10.2.5.1

athātaścayanasyaiva antaropasadau cinotyetadvai devā abibhayuryadvai na imamiha rakṣāṃsi nāṣṭrā na hanyuriti ta etāḥ puro'paśyannupasada imāneva lokānime vai lokāḥ purastāḥ prāpadyanta tāḥ prapadyābhaye'nāṣṭra etamātmānaṃ samaskurvata tathaivaitadyajamāna etāḥ puraḥ prapadyābhaye'nāṣṭra etamātmānaṃ saṃskurute

Now as to the building itself. He builds between the two (performances of the) Upasads. For at that time the gods were afraid lest the fiends, the Rakshas, should destroy that (Agni's body) of theirs (built) there. They saw these strongholds, the Upasads, to wit, these worlds, for these worlds are indeed strongholds. 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 these strongholds, complete this body in a place free from danger and devilry.