Mbh 13.143.2f

Even a Sudra, O goddess, that has purified his soul by pure deeds and that has subjugated all his senses, deserves to be waited upon and served with reverence as a Brahmana. This has been said by the Self-born Brahmana himself. When a pious nature and pious deeds are noticeable in even a Sudra, he should, according to my opinion, be held superior to a person of the three regenerate classes. Neither birth, nor the purificatory rites, nor learning, nor offspring, can be regarded as grounds for conferring upon one the regenerate status. Verily, conduct is the only ground. All Brahmanas in this world are Brahmanas in consequence of conduct. A Sudra, if he is established on good conduct, is regarded as possessed of the status of a Brahmana. The status of Brahma, O auspicious lady, is equal wherever it exists. Even this is my opinion. He, indeed, is a Brahmana in whom the status of Brahma exists,--that condition which is bereft of attributes and which has no stain attached to it. The boon-giving Brahma, while he created all creatures, himself said that the distribution of human beings into the four orders dependent on birth is only for purposes of classification. The Brahmana is a great field in this world,--a field equipped with feet for it moves from place to place. He who plants seeds in that field, O beautiful lady, reaps the crop in the next world. That Brahmana who wishes to achieve his own good should always live upon the remains of the food that may be there in his house after gratifying the needs of all others. He should always adhere to the path of righteousness. Indeed, he should tread along the path that belongs to Brahma. He should live engaged in the study of the Samhitas and remaining at home he should discharge all the duties of a householder. He should always be devoted to the study of the Vedas, but he should never derive the means of subsistence from such study.