Mbh 13.28.1a

"'Bhishma said, "Thus addressed by Indra, Matanga of restrained vows and well regulated soul, (without hearkening to the counsels of the chief of the celestials), stood for a hundred years on one foot, O thou of unfading glory. Sakra of great fame once more appeared before him and addressing him, said,--'The status of a Brahmana, O child, is unattainable. Although thou covetest it, it is impossible for thee to obtain it. O Matanga, by coveting that very high status thou art sure to be destroyed. Do not, O son, betray such rashness. This cannot be a righteous path for thee to follow. O thou of foolish understanding, it is impossible for thee to obtain it in this world. Verily, by coveting that which is unattainable, thou art sure to meet with destruction in no time. I am repeatedly forbidding thee. By striving, however, to attain that high status by the aid of thy penances, notwithstanding my repeated admonition, thou art sure to meet with destruction. From the order of brute life one attains to the status of humanity. If born as human being, he is sure to take birth as a Pukkasa or a Chandala. Verily, one having taken birth in that sinful order of existence, viz., Pukkasa, one, O Matanga, has to wander in it for a very long time. Passing a period of one thousand years in that order, one attains next to the status of a Sudra. In the Sudra order, again, one has to wander for a long time. After thirty thousand years one acquire the status of a Vaisya. There, in that order, one has to pass a very long period. After a time that is sixty times longer than what has been stated as the period of Sudra existence, one becomes a person of the fighting order. In the Kshatriya order one has to pass a very long time. After a time that is measured by multiplying the period last referred to by sixty, one becomes born as a fallen Brahmana. In this order one has to wander for a long period.