Shatapatha Brahmana 6.5.4.1

6.5.4.1

athainamasyāṃ khanati | etadvai devā abibhayuryadvai na imamiha rakṣāṃsi nāṣṭrā na hanyuriti tasmā imāmevātmānamakurvanguptyā ātmātmānaṃ gopsyatīti

He now digs that (hole) in the earth; for the gods now were afraid, thinking, 'We hope the Rakshas, the fiends, will not smite here this (Agni) of ours!' They made this (earth) to be his self (body), for protection, thinking, 'The self will protect itself.'