Mbh 13.10.2a

"'Bhishma said, "In this connection, O king, I shall recite to thee, in due order, what I heard certain Rishis say in days of yore. Instruction should not be imparted unto one that belongs to a low or mean caste. It is said that the preceptor who imparts instruction to such a person incurs great fault. Listen to me, O chief of Bharata's race, as I recite to thee, O Yudhishthira, this instance that occurred in days of old, O monarch, of the evil consequences of the imparting of instruction unto a low-born person fallen into distress. The incident which I shall relate occurred in the asylum of certain regenerate sages that stood on the auspicious breast of Himavat. There, on the breast of that prince of mountains, was a sacred asylum adorned with trees of diverse kinds. Overgrown also with diverse species of creepers and plants, it was the resort of many animals and birds. Inhabited by Siddhas and Charanas also, it was exceedingly delightful in consequence of the woods that flowered these at every season. Many were the Brahmacharins that dwelt there, and many belonging to the forest mode of life. Many also were the Brahmanas that took up their residence there, that were highly blessed and that resembled the sun or the fire in energy and effulgence. Ascetics of diverse kinds, observant of various restraints and vows, as also others, O chief of the Bharatas, that had undergone Diksha and were frugal in fare and possessed of cleansed souls, took up their residence there. Large numbers of Valakhilyas and many that were observant of the vow of Sanyasa also, used to dwell there. The asylum, in consequence of all this, resounded with the chanting of the Vedas and the sacred Mantras uttered by its inhabitants. Once upon a time a Sudra endued with compassion for all creatures, ventured to come into that asylum. Arrived at that retreat, he was duly honoured by all the ascetics.