Śatapatha Brāhmaṇa ("of a hundred paths") 6.3.3.26

6.3.3.26

athainamasyāṃ khanati | etadvai devā abibhayuryadvai na imamahi rakṣāṃsi nāṣṭrā na hanyuriti tasmā imāmevātmānamakurvanguptyā ātmātmānaṃ gopsyatīti sā samambilā syāttadasyeyamātmā bhavati yadveva samaṃbilā yonirvā iyaṃ reta idaṃ yadvai retaso yonimatiricyate 'muyā tadbhavatyatha yannyūnaṃ vyṛddhaṃ tadetadvai retasaḥ samṛddhaṃ yatsamambilaṃ catuḥsraktireṣa bhavati catasro vai diśaḥ sarvābhya evainametaddigbhyaḥ khanati

He then digs for him (Agni) in this earth. For the gods then were afraid, thinking, 'We hope the Rakshas, the fiends, will not smite him here! For the sake of protection they made this earth to be a self (body, âtman) for him, thinking, 'His own self will protect his own self.' It (the lump of clay) should be as large as the hole: thus this earth (or clay) becomes his (Agni's) self. And as to its (being) as large as the hole,--this earth is the womb, and this (clay) is teed; and whatever part of the seed exceeds the womb, becomes useless; and what is deficient, is unsuccessful; but that part of the seed which is within the hole is successful. Four-cornered is this hole, for there are four quarters: from all the (four) quarters he thus digs him.