It comes from the bhAgavata purANa, 12.1.36 or 38, which discusses the lines of kings in kaliyuga. The verse is numbered as 36 or 38 in different versions.
At that time the brāhmaṇas of such provinces as Śaurāṣṭra, Avantī, Ābhīra, Śūra, Arbuda and Mālava will forget all their regulative principles, and the members of the royal order in these places will become no better than śūdras.
Alternative translation- Motilal
The twice-born castes of Śaurāṣṭra, Avantī, Ābhīra, Śūra, Arbuda and Mālava will become fallen (due to the lapse or non-performance of samskaras, such as the investiture of the sacred thread) and the rulers of these people will be mostly of śūdra castes.
Gita Press translation
(As Kaliyuga advances) the Brahmanas of Saurastra, Avanti, the Abhira territory and the principalities of Sura, Arbuda and Malawa will become Vratyas or fallen men (because of their giving up the Samskaras or purificatory rites, especially the ceremony of investiture with the sacred thread), and the rulers will be mostly of the Sudra class.