Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 1.2.3

1.2.3

sa tredhātmānaṃ vyakurutādityaṃ tṛtīyaṃ vāyuṃ tṛtīyam̐ sa eṣa prāṇastredhā vihitaḥ tasya prācī dikśiro'sau cāsau cermau | athāsya pratīcī dikpucchamasau cāsau ca sakthyau dakṣiṇā codīcī ca pārśve dyauḥ pṛṣṭhamantarikṣamudaramiyamuraḥ sa eṣo'psu pratiṣṭhito yatra kva caiti tadeva pratitiṣṭhatyevaṃ vidvān

That being divided itself threefold, Âditya (the sun) as the third, and Vâyu (the air) as the third. That spirit (prâna) became threefold. The head was the Eastern quarter, and the arms this and that quarter (i. e. the N. E. and S. E., on the left and right sides). Then the tail was the Western quarter, and the two legs this and that quarter (i. e. the N. W. and S. W.) The sides were the Southern and Northern quarters, the back heaven, the belly the sky, the dust the earth. Thus he (Mrityu, as arka) stands firm in the water, and he who knows this stands firm wherever he goes.