Srimad-Bhagavatam: Canto 7 - Chapter 1 - Verse 36
Sanskrit:
श्रीनारद उवाचएकदा ब्रह्मण: पुत्रा विष्णुलोकं यदृच्छया ।सनन्दनादयो जग्मुश्चरन्तो भुवनत्रयम् ॥ ३६ ॥
ITRANS:
śrī-nārada uvācaekadā brahmaṇaḥ putrāviṣṇu-lokaṁ yadṛcchayāsanandanādayo jagmuścaranto bhuvana-trayam
Translation:
The great saint Nārada said: Once upon a time when the four sons of Lord Brahmā named Sanaka, Sanandana, Sanātana and Sanat-kumāra were wandering throughout the three worlds, they came by chance to Viṣṇuloka.