Skandapurana 5.164.9

5.164.9

.. ṛṣaya ūcuḥ .. ..
kṣatriyo'pi kathaṃ vipro viśvā mitro mahāmate ..
vasiṣṭhena kathaṃ nokto yaḥ prokto brahmaṇā svayam .. 9 ..

The seers said:
Even though a kṣatriya, how did Viśvāmitra become a brahmin, O noble-minded one? / Why was he, who was proclaimed by Brahmā himself, not spoken to [as such] by Vasiṣṭha? 9