KAṬHA UPANISHAD · 2.3.12

नैव वाचा न मनसा प्राप्तुं शक्यो न चक्षुषा ।
अस्तीति ब्रुवतोऽन्यत्र कथं तदुपलभ्यते ॥ १२॥

naiva vācā na manasā prāptuṃ śakyo na cakṣuṣā ।
astīti bruvato’nyatra kathaṃ tadupalabhyate ॥ 12॥

Not with the mind hath man the power to see God, no, nor by speech nor with the eye. Unless one saith “He is,” how can one become sensible of Him?