Katha Upanishad Verse 1.3.5

If the driver (intellect) lacks discrimination and fails to keep the reins (mind) steady, the horses (senses) become wild and uncontrollable, leading the individual astray.

Sanskrit Verse

यस्त्वविज्ञानवान्भवत्ययुक्तेन मनसा सदा ।
तस्येन्द्रियाण्यवश्यानि दुष्टाश्वा इव सारथेः ॥ ५ ॥

Transliteration

yastvavijñānavānbhavatyayuktena manasā sadā |
tasyendriyāṇyavaśyāni duṣṭāśvā iva sāratheḥ || 5 ||

Translation

But of him who is not possessed of discrimination, and whose mind is always uncontrolled, the senses are not controllable as vicious horses of a driver.