SB 3.8.7

proktaṁ kilaitad bhagavattamena
 nivṛtti-dharmābhiratāya tena
sanat-kumārāya sa cāha pṛṣṭaḥ
 sāṅkhyāyanāyāṅga dhṛta-vratāya
Word for word: 
proktam — was said; kila — certainly; etat — this; bhagavattamena — by Lord Saṅkarṣaṇa; nivṛtti — renunciation; dharma-abhiratāya — unto one who has taken this religious vow; tena — by Him; sanat-kumārāya — unto Sanat-kumāra; saḥ — he; ca — also; āha — said; pṛṣṭaḥ — when inquired of; sāṅkhyāyanāya — unto the great sage Sāṅkhyāyana; aṅga — my dear Vidura; dhṛta-vratāya — unto one who has taken such a vow.
Translation: 
Lord Saṅkarṣaṇa thus spoke the purport of Śrīmad-Bhāgavatam to the great sage Sanat-kumāra, who had already taken the vow of renunciation. Sanat-kumāra also, in his turn, when inquired of by Sāṅkhyāyana Muni, explained Śrīmad-Bhāgavatam as he had heard it from Saṅkarṣaṇa.
Purport: 

This is the way of the paramparā system. Although Sanat-kumāra, the well-known great saintly Kumāra, was in the perfect stage of life, still he heard the message of Śrīmad-Bhāgavatam from Lord Saṅkarṣaṇa. Similarly, when he was questioned by Sāṅkhyāyana Ṛṣi, he spoke to him the same message he had heard from Lord Saṅkarṣaṇa. In other words, unless one hears from the proper authority one cannot become a preacher. In devotional service, therefore, two items out of the nine, namely hearing and chanting, are most important. Without hearing nicely, one cannot preach the message of Vedic knowledge.