Śatapatha Brāhmaṇa ("of a hundred paths") 3.7.2.1

3.7.2.1

yāvato vai vedistāvatī pṛthivī | vajrā vai yūpāstadimāmevaitatpṛthivīmetairvajraiḥ spṛṇute 'syai sapatnānnirbhajati tasmādyūpaikādaśinī bhavati dvādaśa upaśayo bhavati vitaṣṭastaṃ dakṣiṇata upanidadhāti tadyaddvādaśa upaśayo bhavati

Verily, as large as the altar is, so large is the earth. The sacrificial stakes are thunderbolts; and by means of these thunderbolts he obtains possession of this earth, and excludes his enemies from sharing therein. Hence there are eleven stakes, and the twelfth lies aside rough-hewn; he puts it down south (of the altar). The reason why the twelfth lies aside is this.