Srimad-Bhagavatam: Canto 7 - Chapter 1 - Verse 24

Sanskrit:

शूद्रस्य सन्नति: शौचं सेवा स्वामिन्यमायया ।अमन्त्रयज्ञो ह्यस्तेयं सत्यं गोविप्ररक्षणम् ॥ २४ ॥

ITRANS:

śūdrasya sannatiḥ śaucaṁsevā svāminy amāyayāamantra-yajño hy asteyaṁsatyaṁ go-vipra-rakṣaṇam

Translation:

Offering obeisances to the higher sections of society [the brāhmaṇas, kṣatriyas and vaiśyas], being always very clean, being free from duplicity, serving one’s master, performing sacrifices without uttering mantras, not stealing, always speaking the truth and giving all protection to the cows and brāhmaṇas — these are the symptoms of the śūdra.

Purport:

It is everyone’s experience that workers or servants are generally accustomed to stealing. A first-class servant is one who does not steal. Here it is recommended that a first-class śūdra must remain very clean, must not steal or speak lies, and must always render service to his master. A śūdra may attend sacrifices and Vedic ritualistic ceremonies along with his master, but he should not utter the mantras, for these may be uttered only by the members of the higher sections of society. Unless one is completely pure and has been raised to the standard of a brāhmaṇa, kṣatriya or vaiśya — in other words, unless one is dvija, twice-born — the chanting of mantras will not be fruitful.