Kaṭha Upaniṣad 139

139

yas tu punaḥ pūrvoktaviparītasārathir bhavati vijñānavān pragṛhītamanāḥ samāhitacittaḥ sadā, tasyāśvasthānīyānīndriyāṇi pravartayituṃ nivartayituṃ vā śakyāni vaśyāni dāntāḥ sadaśvā ivetarasāratheḥ ///1,3.6 //

Shankara’s Commentary:
Com.—But of him who is a driver, the contrary of one already explained, i.e., of the driver who knows and who has the mind always under restraint, the senses standing in the place of the horses can be let go or stopped, i.e., controlled like the good horses of the other driver.