Katha Upanishad
The Katha Upanishad is the most dramatic of all Upanishads — a young brahmin boy named Nachiketa is sent to the house of Death by his father, waits three days and nights without food, and earns three boons from Yama. The third boon — the secret of death and what lies beyond — forms the philosophical heart of the text. Yama's teaching includes the famous chariot metaphor and the description of the Purusha smaller than the smallest and greater than the greatest.
उशन् ह वै वाजश्रवसः सर्ववेदसं ददौ। तस्य ह नचिकेता नाम पुत्र आस।
तं ह कुमारं सन्तं दक्षिणासु नीयमानासु श्रद्धाऽऽविवेश सोऽमन्यत।
श्रेयश्च प्रेयश्च मनुष्यमेतस्तौ सम्परीत्य विविनक्ति धीरः। श्रेयो हि धीरोऽभि प्रेयसो वृणीते प्रेयो मन्दो योगक्षेमाद् वृणीते॥
न विचिकित्से नचिकेतस्त्वमस्मान्नैषा प्रज्ञा विकर्षणीया। वरं वृणीष्व नचिकेतोन्यदेव मा मोपारोत्सीरमृतस्यैष वर्त्मन्॥
आत्मानं रथिनं विद्धि शरीरं रथमेव तु। बुद्धिं तु सारथिं विद्धि मनः प्रग्रहमेव च॥