Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 1.4.1

1.4.1

ātmaivedamagra āsītpuruṣavidhaḥ so'nuvīkṣya nānyadātmano'paśyatso'hamasmītyagre vyāharattato'haṃnāmābhavattasmādapyetarhyāmantrito'hamayamityevāgra uktvāthānyannāma prabrūte yadasya bhavati sa yatpūrvo'smātsarvasmātsarvānpāpmana auṣattasmātpuruṣa oṣati ha vai sa taṃ yo'smātpūrvo bubhūṣati ya evaṃ veda

In the beginning this was Self alone, in the shape of a person (purusha). He looking round saw nothing but his Self. He first said, 'This is I;' therefore he became I by name. Therefore even now, if a man is asked, he first says, 'This is I,' and then pronounces the other name which he may have. And because before (pûrva) all this, he (the Self) burnt down (ush) all evils, therefore he was a person (pur-usha). Verily he who knows this, burns down every one who tries to be before him.