Mbh 12.63.2a

"'"Listen now, O son of Pandu, to those duties that should be observed in the four modes of life. These should be known by the Kshatriya who desires the members of the three (other) orders (in his kingdom) to strictly adhere to the respective duties of those modes. For a Sudra who is desirous of hearing (such scriptures as are not forbidden in his case),[198] who has accomplished his duties, who has begotten a son, between whom and the superior orders there is not much difference in consequence of the purity of his conduct, all the modes of life have been laid down excepting the observance of universal peacefulness and self-restraint (which are not necessary for him). For a Sudra practising all these duties as also for a Vaisya, O king, and a Kshatriya, the Bhikshu mode of life has been laid down. Having discharged the duties of his order, and having also served the kin, a Vaisya of venerable years, with the king's permission, may betake himself to another mode of life.