Bhagavatapurana 4.24.15

4.24.15

yad uktaṃ pathi dṛṣṭena giriśena prasīdatā /
tad dhyāyanto japantaś ca pūjayantaś ca saṃyatāḥ //

(graciously revealing himself) was met on their way.

14 & 15. Commissioned by their father to add to the population (of the world) they went down into the sea for _Tapas_ and worshipped the Lord of Tapas (Hari) by their _Tapas_ for ten thousand years and (all the time) they with perfect self-control contemplated (the Lord) repeating the sacred mantras taught by God Siva who