Ramayana
पुरा कृतयुगे राजन्ब्राह्मणा वै तपस्विनः । अब्राह्मणस्तदा राजन्न तपस्वी कथञ्चन ॥ 7-74-9 ॥
In olden days, in the Krita age, O king, only the brahmins were ascetics; in that age, O king, no one who was not a brahmin practiced austerity in any way whatsoever.
english translation