Śatapatha Brāhmaṇa ("of a hundred paths") 6.3.3.24

6.3.3.24

yadvevainam parilikhati | etadvai devā abibhayuryadvai na imamiha rakṣāṃsi nāṣṭrā na hanyuriti tasmā etām puram paryaśrayaṃstathaivāsmā ayametām puram pariśrayatyabhryā vajro vā abhrirvajramevāsmā etadabhigoptāraṃ karoti sarvataḥ parilikhati sarvata evāsmā etaṃ vajramabhigoptāraṃ karoti triṣkṛtvaḥ parilikhati trivṛtamevāsmā etaṃ vajramabhigoptāraṃ karoti

And, again, why he draws a line around it;--the gods now were afraid, thinking, 'We hope the Rakshas, the fiends, will not smite here this (Agni) of ours!' They drew that rampart round it; and in like manner does this one now draw that rampart round it,--with the spade, for the spade is the thunderbolt, and he thus makes the thunderbolt its (or his, Agni's) protector. He draws it all round: on every side he thus makes that thunderbolt to be its (or his) protector. Three times he draws a line: that threefold thunderbolt he thus makes to be a protector for him.