At Nārāyaṇa-saras, the second group of sons performed penances in the same way as the first. They bathed in the holy water, and by its touch all the dirty material desires in their hearts were cleansed away. They murmured mantras beginning with oṁkāra and underwent a severe course of austerities.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
तदुपस्पर्शनादेव विनिर्धूतमलाशयाः । जपन्तो ब्रह्म परमं तेपुस्तेऽत्र महत्तपः
tadupasparśanādeva vinirdhūtamalāśayāḥ | japanto brahma paramaṃ tepuste'tra mahattapaḥ
SB 6.5.26