SB 7.1.36

śrī-nārada uvāca
ekadā brahmaṇaḥ putrā
 viṣṇu-lokaṁ yadṛcchayā
sanandanādayo jagmuś
 caranto bhuvana-trayam
Word for word: 
śrī-nāradaḥ uvāca — Śrī Nārada Muni said; ekadā — once upon a time; brahmaṇaḥ — of Lord Brahmā; putrāḥ — the sons; viṣṇu — of Lord Viṣṇu; lokam — the planet; yadṛcchayā — by chance; sanandana-ādayaḥ — Sanandana and the others; jagmuḥ — went; carantaḥ — traveling about; bhuvana-trayam — the three worlds.
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.