ŚVETĀŚVATARA UPANISHAD · 4.20

न संदृशे तिष्ठति रूपम् अस्य न चक्षुषा पश्यति कश्चनैनं ।
हृदा हृदिस्थं मनसा य एनम् एवं विदुर् अमृतास् ते भवन्ति ॥

na saṃdṛśe tiṣṭhati rūpam asya na cakṣuṣā paśyati kaścanainaṃ ।
hṛdā hṛdisthaṃ manasā ya enam evaṃ vidur amṛtās te bhavanti ॥

His form cannot be seen, no one perceives him with the eye. Those who through heart and mind know him thus abiding in the heart, become immortal.