Srimad-Bhagavatam: Canto 5 - Chapter 1 - Verse 3
अजनाभं नामैतद्वर्षं भारतमिति यत आरभ्य व्यपदिशन्ति ॥ ३ ॥
ajanābhaṁ nāmaitad varṣaṁ bhāratam iti yata ārabhya vyapadiśanti.
Formerly this planet was known as Ajanābha-varṣa, but since Mahārāja Bharata’s reign it has become known as Bhārata-varṣa.
This planet was formerly known as Ajanābha because of the reign of King Nābhi. After Bharata Mahārāja ruled the planet, it became celebrated as Bhārata-varṣa.