ŚVETĀŚVATARA UPANISHAD · 3.13

अङ्गुष्ठमात्रः पुरुषो ऽन्तरात्मा सदा जनानां हृदये संनिविष्टः ।
हृदा मनीषा मनसाभिकॢप्तो य एतद् विदुर् अमृतास् ते भवन्ति ॥

aṅguṣṭhamātraḥ puruṣo ’ntarātmā sadā janānāṃ hṛdaye saṃniviṣṭaḥ ।
hṛdā manīṣā manasābhikḷpto ya etad vidur amṛtās te bhavanti ॥

The person (purusha), not larger than a thumb, dwelling within, always dwelling in the heart of man, is perceived by the heart, the thought, the mind; they who know it become immortal.