Ramayana
एवं विदेहराजश्च जनकः पूर्वको ऽभवत् । मिथिर्नाम महातेजास्तेनायं मैथिलो ऽभवत् ॥ 7-57-20 ॥
Thus Janaka became the first king of the Videhas; he of great splendor was named Mithi, and by him this lineage became known as Maithila.
english translation