Mahabharata 12.322.51

उत्पन्नेऽऽङ्गिरसे चैव युगे प्रथमकल्पितेसाङ्गोपनिषदं शास्त्रं स्थापयित्वा बृहस्पतौ

utpanne''ṅgirase caiva yuge prathamakalpitesāṅgopaniṣadaṃ śāstraṃ sthāpayitvā bṛhaspatau

Our plain-language meaning of this verse is waiting for scholar review.