Śatapatha Brāhmaṇa ("of a hundred paths") 13.8.4.2

13.8.4.2

atha dakṣiṇataḥ parivakre khananti te kṣīreṇa codakena ca pūrayanti te hainamamuṣmiṃlokeṛkṣite kulye upadhāvataḥ saptottarastā udakena pūrayanti na ha vai sapta sravantīraghamatyetumarhatyaghasyaivānatyayāya

On the right (south) side they then dig two somewhat curved (furrows), and fill them with milk and water,--these, indeed, are two inexhaustible streams (that) flow to him in the other world;--and seven (they dig) on the left (north) side, and fill them with water, for sin not to pass beyond, for indeed sin cannot pass beyond seven rivers.