Katha Upanishad Verse 2.3.9

The Atman cannot be perceived by the physical eyes. It is revealed only through the intellect controlling the mind and by constant, focused meditation. Those who know this truth become immortal.

Sanskrit Verse

न संदृशे तिष्ठति रूपमस्य न चक्शुषा पश्यति कश्चनैनम् ।
हृदा मनीषा मनसाभिक्लृप्तो य एतद्विदुरमृतास्ते भवन्ति ॥ ९ ॥

Transliteration

na saṃdṛśe tiṣṭhati rūpamasya na cakśuṣā paśyati kaścanainam |
hṛdā manīṣā manasābhiklṛpto ya etadviduramṛtāste bhavanti || 9 ||

Translation

His form stands not within the fold of vision. None sees him with the eye. By the intellect controlling the mind, and by constant meditation is he revealed. Whoso knows that becomes immortal.