Mahabharata 13.131.4

केन वा कर्मणा विप्रः शूद्रयोनौ प्रजायतेक्षत्रियः शूद्रतामेति केन वा कर्मणा विभो

kena vā karmaṇā vipraḥ śūdrayonau prajāyatekṣatriyaḥ śūdratāmeti kena vā karmaṇā vibho

Our plain-language meaning of this verse is waiting for scholar review.