Mbh 13.27.2a

"'Bhishma said, "The status of a Brahmana, O Yudhishthira, is incapable of acquisition by a person belonging to any of the three other orders. That status is the highest with respect to all creatures. Travelling through innumerable orders of existence, by undergoing repeated births, one at last, in some birth, becomes born as a Brahmana. In this connection is cited an old history, O Yudhishthira, of a conversation between Matanga and a she-ass. Once on a time a Brahmana obtained a son who, though procreated by a person belonging to a different order, had, however, the rites of infancy and youth performed in pursuance of the ordinances laid down for Brahmanas. The child was called by the name of Matanga and was possessed of every accomplishment. His father, desiring to perform a sacrifice, ordered him, O scorcher of foes, to collect the articles required for the act. Having received the command of his father, he set out for the purpose, riding on a car of great speed, drawn by an ass. It so happened that the ass yoked unto that car was of tender years. Instead therefore, of obeying the reins, the animal bore away the car to the vicinity of its dam, viz., the she-ass that had brought it forth. Matanga, dissatisfied with this, began to strike repeatedly the animal with his goad on its nose. Beholding those marks of violence on her child's nose, the she-ass, full of affection for him, said--'Do not grieve, O child, for his treatment. A Chandala it is that is driving thee. There is no severity in a Brahmana. The Brahmana is said to be the friend of all creatures. He is the teacher also of all creatures and their ruler. Can he chastise any creature so cruelly? This fellow, however, is of sinful deeds. He hath no compassion to show unto even a creature of such tender years as thou. He is simply proving the order of his birth by conducting himself in this way.