Śatapatha Brāhmaṇa ("of a hundred paths") 3.7.2.2

3.7.2.2

devā ha vai yajñaṃ tanvānāḥ | te 'surarakṣasebhya āsaṅgādbibhayāṃ cakrustadya eta ucritā yatheṣurastā tayā vai stṛṇute vā na vā stṛṇute yathā daṇḍaḥ prahṛtastena vai stṛṇute vā na vā stṛṇute 'tha ya eṣa dvādaśa upaśayo bhavati yatheṣurāyatānastā yathodyatamaprahṛtamevameṣa vajra udyato dakṣiṇato nāṣṭrāṇāṃ rakṣasāmapahatyai tasmāddvādaśa upaśayo bhavati

Now the gods, while performing this sacrifice, were afraid of an attack from the Asura-Rakshas. Those raised (sacrificial stakes), then, were as a discharged arrow,--therewith one either smites or smites not; as a hurled club,--therewith one either smites or smites not. But that twelfth (stake) lying aside,--even as an arrow drawn but not discharged, as (a weapon) raised but not hurled, so was that a thunderbolt raised for repelling the evil spirits on the south; therefore the twelfth (stake) lies aside.