BṚHADĀRAṆYAKA UPANISHAD · 2.5.18

इदं वै तन् मधु दध्यङ्ङ् आथर्वणो ऽश्विभ्याम् उवाच ।
तद् एतद् ऋषिः पश्यन्न् अवोचत् -- पुरश् चक्रे द्विपदः पुरश् चक्रे चतुष्पदः ।
पुरः स पक्षी भुत्वा पुरः पुरुष आविशद् इति ।
स वा अयं पुरुषः सर्वासु पूर्षु पुरिशयः ।
नैनेन किं चनानावृतम् ।
नैनेन किं चनासंवृतम् ॥

idaṃ vai tan madhu dadhyaṅṅ ātharvaṇo ’śvibhyām uvāca ।
tad etad ṛṣiḥ paśyann avocat -- puraś cakre dvipadaḥ puraś cakre catuṣpadaḥ ।
puraḥ sa pakṣī bhutvā puraḥ puruṣa āviśad iti ।
sa vā ayaṃ puruṣaḥ sarvāsu pūrṣu puriśayaḥ ।
nainena kiṃ canānāvṛtam ।
nainena kiṃ canāsaṃvṛtam ॥

Verily Dadhyak Âtharvana proclaimed this honey to the two Asvins, and a Rishi, seeing this, said: 'He (the Lord) made bodies with two feet, he made bodies with four feet. Having first become a bird, he entered the bodies as purusha (as the person).' This very purusha is in all bodies the purisaya, i.e. he who lies in the body (and is therefore called purusha). There is nothing that is not covered by him, nothing that is not filled by him.

F. Max Müller · The Upanishads, Sacred Books of the East 15 (1884)