Mahabharata Anushasna Parva Chapter 143:4

Mahabharata Anushasna Parva (Dana Dharma Parva) Chapter 143:4 He should never indulge in sexual pleasure, but live cheerfully and in independence, well-conversant with the science of Wealth or Profit. Of righteous soul, he should seek his wedded spouse only in her season. He should always observe fasts, keep his soul under control, devote himself to the study of the Vedas, and be pure in body and mind. He should sleep on blades of Kusa grass spread out in his fire chamber. He should pursue the aggregate of Three (viz., Righteousness, Wealth, and Pleasure), and be always cheerful. Unto Sudras desirous of food, he should always answer that it is ready. He should never desire any thing from motives of gain or pleasure. He should worship the Pitris and gods and guests. In his own house he should live the life of a mendicant. He should duly adore the deities in his Agnihotra, morning, noon, and evening every day, by pouring libations agreeably to the ordinance. With his face turned towards the foe, he should cast off his life-breath in battle fought for the benefit of kine and Brahmanas. Or he may enter the triple fires sanctified with Mantras and cast off his body. By pursuing this line of conduct he takes birth in his next life as a Brahmana. Endued with knowledge and science, purified from all dross, and fully conversant with the Vedas, a pious Kshatriya, by his own acts, becomes a Brahmana. It is with the aid of these acts, O goddess, that a person who has sprung from a degraded order, viz., a Sudra, may become a Brahmana refined of all stains and possessed of Vedic lore, One that is a Brahmana, when he becomes wicked in conduct and observes no distinction in respect of food, falls away from the status of Brahmanahood and becomes a Sudra. 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.