Mbh 12.328.2a

"'"The disciples said, 'We have, through thy grace, been endued with great energy. Our fame also has spread. There is one favour that we humbly solicit thee to grant us.' Hearing these words of theirs, the regenerate Rishi answered them, saying, 'Ye sons, tell me what that boon is which ye wish I should grant you!' Hearing this answer of their preceptor, the disciples became filled with joy. Once more bowing their heads low unto their preceptor and joining their hands, all of them in one voice said, O king, these excellent words: 'If our preceptor has been pleased with us, then, O best of sages, we are sure to be crowned with success! We all solicit thee, O great Rishi, to grant us a boon. Be thou inclined to be graceful to us. Let no sixth disciple (besides us five) succeed in attaining to fame! We are four. Our preceptor's son forms the fifth. Let the Vedas shine in only us five! Even this is the boon that we solicit.'--Hearing these words of his disciples, Vyasa, the son of Parasara, possessed of great intelligence, well-conversant with the meaning of the Vedas, endued with a righteous soul, and always engaged in thinking of objects that confer benefits on a person in the world hereafter, said unto his disciples these righteous words fraught with great benefit: 'The Vedas should always be given unto him who is a Brahmana, or unto him who is desirous of listening to Vedic instructions, by him who eagerly wishes to attain a residence in the region of Brahman! Do ye multiply. Let the Vedas spread (through your exertions). The Vedas should never be imparted unto one that has not formally become a disciple. Nor should they be given unto one who is not observant of good vows. Nor should they be given for dwelling in one that is of uncleansed soul. These should be known as the proper qualifications of persons that can be accepted as disciples (for the communication of Vedic knowledge).