Skanda Purana 4.101.21

4.101.21

19 pituraṃkādhiṣṭhitasya sāpatnamātrā tiraskṛtasya dhruvasya mātṛvacanāttapasyārthaṃ vipinapraveśaḥ, tatra saptarṣisamāgamaḥ, tasmai dvādśākṣaramantramupadiśya munīnāmantardhānaṃ ca. 82 1

19. Dhruva, seated on his father’s lap, is rebuked by his stepmother; at his mother's word he enters the forest to perform tapas; there he meets the Seven Seers; the munis instruct him in the twelve-syllable mantra and disappear. 82 1