Mbh 12.328.2b

No science should be imparted unto one without a proper examination of one's character, as pure gold is tested by heat, cutting and rubbing, after the same manner disciples should be tested by their birth and accomplishments. Ye should never set your disciples to tasks to which they should not be set, or to tasks that are fraught with danger. One's knowledge is always commensurate with one's understanding and diligence in study. Let all disciples conquer all difficulties, and let all of them meet with auspicious success. Ye are competent to lecture on the scriptures unto persons of all the orders. Only ye should, while lecturing, address a Brahmana, placing him in the van. These are the rules in respect of the study of the Vedas. This again is regarded as a high task. The Vedas were created by the Self-born for the purpose of praising the deities therewith. That man who, through stupefaction of intellect, speaks ill of a Brahmana well-conversant with the Vedas, is certain to meet with humiliation in consequence of such evil-speaking. He who disregarding all righteous rules, solicits knowledge, and he who, disregarding the rules of righteousness, communicates knowledge, either of them falls off and instead of that affection which should prevail between preceptor and disciple, such questioning and such communication are sure to produce distrust and suspicion. I have now told ye everything about the way in which the Vedas should be studied and taught. Ye should act in this way towards your disciples, bearing these instructions in your minds.'"'"