Mbh 13.143.2c

That Brahmana who drinks alcohol, who becomes guilty of Brahmanicide or mean in his behaviour, or a thief, or who breaks his vows, or becomes impure, or unmindful of his Vedic studies, or sinful, or characterised by cupidity, or guilty of cunning or cheating, or who does not observe vows, or who weds a Sudra woman, or who derives his subsistence by pandering to the lusts of other people or who sells the Soma plant, or who serves a person of an order below his, falls away from his status of Brahmanahood.[572] That Brahmana who violates the bed of his preceptor, or who cherishes malice towards him, or who takes pleasure in speaking ill of him, falls away from the status of Brahmanahood even if he be conversant with Brahman. By these good acts, again, O goddess, when performed, a Sudra becomes a Brahmana, and a Vaisya becomes a Kshatriya. The Sudra should perform all the duties laid down for him, properly and according to the ordinance. He should always wait, with obedience and humility, upon person of the three other orders and serve them with care. Always adhering to the path of righteousness, the Sudra should cheerfully do all this. He should honour the deities and persons of the regenerate orders. He should observe the vow of hospitality to all persons. With senses kept under subjection and becoming abstemious in food, he should never approach his wife except in her season. He should ever search after persons that are holy and pure. As regards food, he should eat that which remains after the needs of all persons have been satisfied. If, indeed, the Sudra desires to be a Vaisya (in his next life), he should also abstain from meat of animals not slain in sacrifices. If a Vaisya wishes to be a Brahmana (in his next life), he should observe even these duties. He should be truthful in speech, and free from pride or arrogance. He should rise superior to all pairs of opposites (such as heat and cold, joy and sorrow, etc.)