Katha-1-3-6-yastu vijñāna

Sri Shankara’s Commentary (Bhashya) translated by S. Sitarama Sastri

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.

Links
Various resources on the Kathopanishad
Upanishad Series – Katopanishad Commentary by Swami Chinmayananda