Srimad Bhagavatam Canto 05, Chapter 18, Text 34

SB 5.18.34

uttaresu ca kurusu bhagavan yajña-purusah krta-varaha-rupa aste tam tu devi haisa bhuh saha kurubhir askhalita-bhakti-yogenopadhavati imam ca paramam upanisadam avartayati.
 
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
Sukadeva Gosvami said: Dear King, the Supreme Lord in His boar incarnation, who accepts all sacrificial offerings, lives in the northern part of Jambudvipa. There, in the tract of land known as Uttarakuru-varsa, mother earth and all the other inhabitants worship Him with unfailing devotional service by repeatedly chanting the following Upanisadic mantra.
Srimad Bhagavatam Canto 05, Chapter 18, Text 33
Srimad Bhagavatam Canto 05, Chapter 18, Text 35