KAṬHA UPANISHAD · 1.3.3

आत्मानँ रथिनं विद्धि शरीरँ रथमेव तु ।
बुद्धिं तु सारथिं विद्धि मनः प्रग्रहमेव च ॥ ३॥

ātmānam̐ rathinaṃ viddhi śarīram̐ rathameva tu ।
buddhiṃ tu sārathiṃ viddhi manaḥ pragrahameva ca ॥ 3॥

Know the body for a chariot and the soul for the master of the chariot: know Reason for the charioteer and the mind for the reins only.