Shatapatha Brahmana 8.5.1.13

8.5.1.13

tām prācīṃ tiraścīmupadadhāti | etaddhaitayā prajāpatiḥ pāpmano mūlamavṛścattathaivainayāyametatpāpmano mūlaṃ vṛścati dakṣiṇato dakṣiṇataudyāmo hi vajro 'ntareṇa dakṣiṇāṃ diśyāmudyāmāya ha tamavakāśaṃ karoti

This one he lays down with its line-marks running eastward and crosswise; for by that one Pragâpati then cut out the root of evil, and in like manner does this (Sacrificer) now thereby cut out the root of evil. On the right (south) side (from the centre he places it), for the thunderbolt has a string on the right side;--inside the one in the southern quarter, for it is for the sake of extension that he leaves that space.