Ramayana
त्रिभ्यो युगेभ्यस्त्रीन्वर्णान्धर्मश्च परिनिष्ठितः । न शुद्धो लभते धर्मं युगतस्तु नरर्षभ । हीनवर्णो नृपश्रेष्ठ तप्यते सुमहत्तपः ॥ 7-74-26 ॥
Through three ages, O bull among men, dharma has been firmly established for the three varnas; and by reason of the order of the ages a Shudra may not lawfully obtain the practice of dharma. In this age, O best of kings, a man of low varna who performs exceedingly great penance commits a transgression.
english translation