Mbh 13.10.10a

"'Bhishma continued, "Dismissed by the king (from the office of priest), the Brahmana made many gifts, unto persons of his own order, of wealth and land and villages. He observed many rigid and severe vows as laid down by the foremost of Brahmanas. He sojourned to many sacred waters and made many gifts unto Brahmanas in those places. Making gifts of kine unto persons of the regenerate order, his soul became cleansed and he succeeded in acquiring a knowledge of it. Repairing to that very asylum whither he had lived in his former birth, he practised very severe penances. As the consequence of all this, O foremost of kings, that Brahmana succeeded in attaining to the highest success. He became an object of veneration with all the ascetics that dwelt in that asylum. In this way, O best of monarchs, that regenerate Rishi fell into great distress. Unto Sudras, therefore, the Brahmanas should never give instructions. Hence, O king, the Brahmana should avoid imparting instructions (to such as are low-born), for it was by imparting instruction to a low-born person a Brahmana came to grief. O best of kings, the Brahmana should never desire to obtain instruction from, or impart instruction to, a person that belongs to the lowest order. Brahmanas and Kshatriyas and Vaisyas, the three orders, are regarded as twice-born. By imparting instruction unto these, a Brahmana does not incur any fault. They, therefore, that are good, should never discourse on any subject, for imparting any instruction, before persons of the inferior order. The course of morality is exceedingly subtile and incapable of being comprehended by persons of uncleansed souls. It is for this reason that ascetics adopt the vow of silence, and being respected by all, pass through Diksha (initiation) without indulging in speech.[26] For fear of saying what is incorrect or what may offend, ascetics often forego speech itself.