SB 7.1.36
sri-narada uvaca
ekada brahmanah putra
visnu-lokam yadrcchaya
sanandanadayo jagmus
caranto bhuvana-trayam
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The great saint Narada said: Once upon a time when the four sons of Lord Brahma named Sanaka, Sanandana, Sanatana and Sanat-kumara were wandering throughout the three worlds, they came by chance to Visnuloka.