MUṆḌAKA UPANISHAD · 3.1.8

न चक्षुषा गृह्यते नापि वाचा
नान्यैर्देवैस्तपसा कर्मणा वा ।
ज्ञानप्रसादेन विशुद्धसत्त्वस्ततस्तु तं पश्यते निष्कलं ध्यायमानः ॥ ८॥

na cakṣuṣā gṛhyate nāpi vācā
nānyairdevaistapasā karmaṇā vā ।
jñānaprasādena viśuddhasattvastatastu taṃ paśyate niṣkalaṃ dhyāyamānaḥ ॥ 8॥

Eye cannot seize, speech cannot grasp Him, nor these other godheads; not by austerity can he be held nor by works: only when the inner being is purified by a glad serenity of knowledge, then indeed, meditating, one beholds the Spirit indivisible.

Sri Aurobindo · CWSA 18, p. 143