Bhagavatapurana 1.3.11

1.3.11

ṣaṣṭham atrerapatyatvaṃ vṛtaḥ prāpto 'nasūyayā /
ānvīkṣikīm alarkāya prahlādādibhya ūcivān //

11. In the sixth He became Dattatreya, the son of Atri, as prayed for by him, without finding fault with the Rishi (for such a prayer), and taught Atmavidya (the true nature of the soul, etc.,) to King Alarka, Prahlada and other righteous men.