Śatapatha Brāhmaṇa ("of a hundred paths") 1.4.4.8

1.4.4.8

devā ha vai yajñaṃ tanvānāḥ | te 'surarakṣasebhya āsaṅgādbibhayāṃ cakrusta etaddakṣiṇataḥ pratyudaśrayannucritamiva hi vīryaṃ tasmāddakṣiṇatastiṣṭhannāghārayati sa yadubhayata āghārayati tasmādidam manaśca vākca samānameva sannāneva śiro ha vai yajñasyaitayoranyatara āghārayormūlamanyataraḥ

Now the gods, when they were performing sacrifice, were afraid of a disturbance on the part of the Asuras and Rakshas. They, therefore, stood up erect against them on the south side (of the sacrificial ground); for strength is, as it were, erect; hence he makes the (second) libation while standing to the south (of the fire). When he makes a libation on each side (of the fire, north and south), this (pair), mind and speech, though indeed joined together, become separate: for one of the two libations is the head of the sacrifice and the other is its root.