Kaṭha Upaniṣad 155

155

uttiṣṭhata jāgrata prāpya varān nibodhata /
kṣurasya dhārā niśitā duratyayā durgaṃ pathas tat kavayo vadanti ///1,3.14 //

14. Arise, awake; having reached the great, learn; the edge of a razor is sharp and impassable; that path, the intelligent say, is hard to go by.