Katha Upanishad - Verse 2.3.12
Text: नैव वाचा न मनसा प्राप्तुं शक्यो न चक्शुषा । अस्तीति ब्रुवतोऽन्यत्र कथं तदुपलभ्यते ॥ १२ ॥ naiva vācā na manasā prāptuṃ śakyo na cakśuṣā | astīti bruvato’nyatra kathaṃ tadupalabhyate || 12 || 12. Not by speech, not by mind, not by the eye, can he be attained; except in his case who says ‘He is,’ how can that be known. Shankara’s Commentary: If the Brahman could be perceived by the operation of the intellect, etc....