BṚHADĀRAṆYAKA UPANISHAD · 1.4.13

स नैव व्यभवत् ।
स शौद्रं वर्णम् असृजत पूषणम् ।
इयं वै पूषा ।
इयं हीदं सर्वं पुष्यति यद् इदं किञ्च ॥

sa naiva vyabhavat ।
sa śaudraṃ varṇam asṛjata pūṣaṇam ।
iyaṃ vai pūṣā ।
iyaṃ hīdaṃ sarvaṃ puṣyati yad idaṃ kiñca ॥

He was not strong enough. He created the Sûdra colour (caste), as Pûshan (as nourisher). This earth verily is Pûshan (the nourisher); for the earth nourishes all this whatsoever.

F. Max Müller · The Upanishads, Sacred Books of the East 15 (1884)