Śrīmad-Bhāgavatam · Text 3.8.7
Who is speaking and story frame
प्रोक्तं किलैतद्भगवत्तमेन
निवृत्तिधर्माभिरताय तेन ।
सनत्कुमाराय स चाह पृष्ट:
सांख्यायनायाङ्ग धृतव्रताय ॥ ७ ॥
proktaṁ kilaitad bhagavattamena
nivṛtti-dharmābhiratāya tena
sanat-kumārāya sa cāha pṛṣṭaḥ
sāṅkhyāyanāyāṅga dhṛta-vratāya
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.
Read Śrīla Prabhupāda's purport on Vedabase.io