Mantra 9

हिरण्मये परे कोशे विरजं ब्रह्म निष्कलम् ।तच्छुभ्रं ज्योतिषां ज्योतिस्तद्यदात्मविदो विदुः ॥ ९ ॥

In plain words

In the supreme golden kośa is Brahman, stainless and without parts. That pure light of lights is what the Ātmavids know.