Atharvaveda 5.6.1

5.6.1

bráhma jajñānáṃ prathamáṃ purástād ví sīmatáḥ surúco vená āvaḥ |
sá budhnyā̀ upamā́ asya viṣṭhā́ḥ satáś ca yónim ásataś ca ví vaḥ ||1||

The bráhman that was first born of old, Vena hath unclosed from the well-shining edge; he unclosed the fundamental nearest positions of it, the womb of the existent and of the non-existent.