KAṬHA UPANISHAD · 2.3.17

अङ्गुष्ठमात्रः पुरुषोऽन्तरात्मा
सदा जनानां हृदये संनिविष्टः ।
तं स्वाच्छरीरात्प्रवृहेन्मुञ्जादिवेषीकां धैर्येण ।
तं विद्याच्छुक्रममृतं तं विद्याच्छुक्रममृतमिति ॥ १७॥

aṅguṣṭhamātraḥ puruṣo’ntarātmā
sadā janānāṃ hṛdaye saṃniviṣṭaḥ ।
taṃ svāccharīrātpravṛhenmuñjādiveṣīkāṃ dhairyeṇa ।
taṃ vidyācchukramamṛtaṃ taṃ vidyācchukramamṛtamiti ॥ 17॥

The Purusha, the Spirit within, who is no larger than the finger of a man is seated for ever in the heart of creatures; one must separate Him with patience from one’s own body as one separates from a blade of grass its main fibre. Thou shalt know Him for the Bright Immortal, yea, for the Bright Immortal.

Sri Aurobindo · CWSA 18, pp. 126, 127