Mahabharata 3.205.19

दुर्ज्ञेयः शाश्वतो धर्मः शूद्रयोनौ हि वर्ततान त्वां शूद्रमहं मन्ये भवितव्यं हि कारणम्येन कर्मविपाकेन प्राप्तेयं शूद्रता त्वया

durjñeyaḥ śāśvato dharmaḥ śūdrayonau hi vartatāna tvāṃ śūdramahaṃ manye bhavitavyaṃ hi kāraṇamyena karmavipākena prāpteyaṃ śūdratā tvayā

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