Ramayana
भविष्यच्छूद्रयोन्यां वै तपश्चर्या कलौ युगे । अधर्मः परमो राजन्द्वापरे शूद्रजन्मनः ॥ 7-74-27 ॥
The practice of austerity by one born of a Shudra womb is destined for the Kali age; but in the Dvapara age, O king, austerity by one of Shudra birth is the greatest unrighteousness.
english translation