ŚVETĀŚVATARA UPANISHAD · 5.6

तद् वेदगुह्योपनिषत्सु गूढं तद् ब्रह्मा वेदते ब्रह्मयोनिं ।
ये पूर्वं देवा ऋषयश् च तद् विदुस् ते तन्मया अमृता वै बभूवुः ॥

tad vedaguhyopaniṣatsu gūḍhaṃ tad brahmā vedate brahmayoniṃ ।
ye pūrvaṃ devā ṛṣayaś ca tad vidus te tanmayā amṛtā vai babhūvuḥ ॥

Brahma (Hiranyagarbha) knows this, which is hidden in the Upanishads, which are hidden in the Vedas, as the Brahma-germ. The ancient gods and poets who knew it, they became it and were immortal.