Mbh 13.135.2a

"'Bhishma said, "A Brahmana may take his food from another Brahmana or from a Kshatriya or a Vaisya, but he must never accept food from a Sudra. A Kshatriya may take his food from a Brahmana, a Kshatriya or a Vaisya. He must, however, eschew food given by Sudras who are addicted to evil ways and who partake of all manner of food without any scruple. Brahmanas and Kshatriyas can partake of food given by such Vaisyas as tend the sacred fire every day, as are faultless in character, and as perform the vow of Chaturmasya. But the man who takes food from a Sudra, swallows the very abomination of the earth, and drinks the excretions of the human body, and partakes of the filth of all the world. He partakes of the very filth of the earth who takes his food thus from a Sudra. Verily, those Brahmanas that take their food from Sudras, take the dirt of the earth. If one engages in the service of a Sudra, one is doomed to perdition though one may duly perform all the rites of one's order. A Brahmana, a Kshatriya, or a Vaisya, so engaging, is doomed, although devoted to the due performance of religious rites. It is said that a Brahmana's duty consists in studying the Vedas and seeking the welfare of the human race; that a Kshatriya's duty consists in protecting men, and that a Vaisya's in promoting their material prosperity. A Vaisya lives by distributing the fruits of his own acts and agriculture. The breeding of kine and trade are the legitimate work in which a Vaisya may engage without fear of censure. The man who abandons his own proper occupation and betakes himself to that of a Sudra, should be considered as a Sudra and on no account should any food be accepted from him. Professors of the healing art, mercenary soldiers, the priest who acts as warder of the house, and persons who devote a whole year to study without any profit, are all to be considered as Sudras.