Mahabharata 12.306.108

उपनिषदमुपाकरोत्तदा वै; जनकनृपस्य पुरा हि याज्ञवल्क्यःयदुपगणितशाश्वताव्ययं त;च्छुभममृतत्वमशोकमृच्छतीति

upaniṣadamupākarottadā vai; janakanṛpasya purā hi yājñavalkyaḥyadupagaṇitaśāśvatāvyayaṃ ta;cchubhamamṛtatvamaśokamṛcchatīti

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