Bhagavatapurana 5.18.34

5.18.34

uttareṣu ca kuruṣu bhagavān yajña-puruṣaḥ kṛta-varāha-rūpa āste taṃ tu devī haiṣā bhūḥ saha kurubhir askhalita-bhakti-yogenopadhāvati imāṃ ca paramām upaniṣadam āvartayati //

34. “In the region of north Kurus, the glorious Lord worshipped in sacrifices presents Himself in the form of *Varaha* whom this divine *Bhu* (*Earth*) along with all the Kuru peoples worships in constant devotion, and she repeats this great *Upanishad* (most sacred and secret prayer).