Śatapatha Brāhmaṇa ("of a hundred paths") 2.6.1.9

2.6.1.9

sa dakṣiṇaiva dṛṣadupale upadadhāti | dakṣiṇārdhe gārhapatyasya ṣaṭpālānyupadadhāti tadyadetāṃ dakṣiṇāṃ diśaṃ sacanta eṣā hi dik pitṝṇāṃ tasmādetāṃ dakṣiṇāṃ diśaṃ sacante

He places the two mill-stones on (the black antelope skin, so as to be inclined) towards the south; and puts the six potsherds on the south part of the Gârhapatya hearth. The reason why they keep the southern direction is because that is the region of the fathers: this is why they keep the southern direction.