Praśna Upaniṣad ("Six Questions") 6.5

6.5

evamevāsya paridraṣṭurimāḥ ṣoḍaśakalāḥ puruṣāyaṇāḥ puruṣaṃ prāpyāstaṃ gacchanti bhidyete cāsāṃ nāmarūpe puruṣa ityevaṃ procyate sa eṣo'kalo'mṛto bhavati
tadeṣa ślokaḥ

As these flowing rivers that go towards the ocean, when they have reached the ocean, sink into it, their name and form are broken, and people speak of the ocean only, exactly thus these sixteen parts of the spectator that go towards the person (purusha), when they have reached the person, sink into him, their name and form are broken, and people speak of the person only, and he becomes without parts and immortal. On this there is this verse: