Mbh 12.63.1a

"'Bhishma said, "Drawing the bow-string, destruction of foes, agriculture, trade, tending cattle, and serving others for wealth, these are improper for a Brahmana. An intelligent Brahmana, leading a domestic mode of life, should duly perform the six Vedic acts. The retirement of a Brahmana into the woods, after having duly discharged all the duties of the domestic mode of life, is applauded. A Brahmana should avoid service of the king, wealth obtained by agriculture, sustenance derived from trade, all kinds of crooked behaviour, companionship with any but his wedded wives, and usury. That wretched Brahmana who falls away from his duties and whose behaviour becomes wicked, becomes, O king, a Sudra. The Brahmana who weds a Sudra woman, who becomes vile in conduct or a dancer or a village servant or does other improper acts, becomes a Sudra. Whether he recites the Vedas or not, O king, if he does such improper acts, he becomes equal to a Sudra and on occasions of feeding he should be assigned a place amongst Sudras. Such Brahmanas become equal to Sudras, O king, and should be discarded on occasions of worshipping the Gods.[197] Whatever presents of food dedicated to the gods and the Pitris are made unto Brahmanas that have transgressed all restraints or become impure in behaviour or addicted to wicked pursuits and cruel acts or fallen away from their legitimate duties, confer no merit (on the giver). For this reason, O king, self-restraint and purity and simplicity have been laid down as the duties of a Brahmana. Besides these, O monarch, all the four modes of life were laid down by Brahman for him. He that is self-restrained, has drunk the Soma in sacrifices, is of good behaviour, has compassion for all creatures and patience to bear everything, has no desire of bettering his position by acquisition of wealth, is frank and simple, mild, free from cruelty, and forgiving, is truly a Brahmana and not he that is sinful in acts.