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.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
पञ्चषड्ढायनार्भाभाः पूर्वेषामपि पूर्वजाः । दिग्वाससः शिशून् मत्वा द्वाःस्थौ तान् प्रत्यषेधताम्
pañcaṣaḍḍhāyanārbhābhāḥ pūrveṣāmapi pūrvajāḥ | digvāsasaḥ śiśūn matvā dvāḥsthau tān pratyaṣedhatām
SB 7.1.36