Katha Upanishad Verse 2.1.13

The Purusha (Self), residing in the heart, is likened to a pure light without a trace of smoke. He is the master of past and future, remaining the same today and forever. This Purusha is likened to a pure flame without smoke. To know this light is to transcend fear and recognize that the Soul remains unchanged “today and tomorrow.”

Sanskrit Verse

अङ्गुष्ठमात्रः पुरुषो ज्योतिरिवाधूमकः ।
ईशानो भूतभव्यस्य स एवाद्य स उ श्वः । एतद्वै तत् ॥ १३ ॥

Transliteration

aṅguṣṭhamātraḥ puruṣo jyotirivādhūmakaḥ |
īśāno bhūtabhavyasya sa evādya sa u śvaḥ | etadvai tat || 13 ||

Translation

That Puruṣa, of the size of a thumb, is like a light without smoke, lord of the past and the future. He is the same today and tomorrow. This verily is That.