Katha Upanishad - Verse 1.3.12
Text: एष सर्वेषु भूतेषु गूढोऽऽत्मा न प्रकाशते । दृश्यते त्वग्र्यया बुद्ध्या सूक्श्मया सूक्श्मदर्शिभिः ॥ १२ ॥ eṣa sarveṣu bhūteṣu gūḍho’’tmā na prakāśate | dṛśyate tvagryayā buddhyā sūkśmayā sūkśmadarśibhiḥ || 12 || This Ātman (Self), hidden in all beings, does not shine forth; but It is seen by subtle seers through keen and subtle understanding. NOTE: Tranlsation and commentary is missing from the book.