Śānti-parva Adhyāya 30: Nārada–Parvata Samaya-bhaṅga, Śāpa, and the Marriage of Sukumārī
सूंजयं श्वैत्यमभ्येत्य राजानमिदमूचतु: । वे दोनों लोकपूजित महर्षि “तथास्तु” कहकर पूर्वोक्त प्रतिज्ञा करनेके पश्चात् श्वैतपुत्र राजा सूंजयके पास जाकर इस प्रकार बोले--
sūñjayaṃ śvaityam abhyetya rājānam idam ūcatuḥ | te dvau lokapūjitau maharṣī “tathāstu” kahkar pūrvokta-pratijñāṃ kṛtvā śvaitaputra-rājā sūñjayasya samīpaṃ gatvā evaṃ prāha ||
Having first assented with the words “Tathāstu—So be it,” and having undertaken the vow previously declared, the two great sages, revered by the worlds, approached King Sūñjaya, son of Śvaiti, and spoke as follows.
श्रीकृष्ण उवाच
The verse highlights the dharmic seriousness of assent and vow-taking: when revered sages say “tathāstu” and undertake a pratijñā, their word becomes a moral commitment that guides subsequent action and lends legitimacy to what follows.
Two world-honoured sages, after agreeing (“tathāstu”) and making the earlier-mentioned pledge, approach King Sūñjaya (son of Śvaiti) and begin to address him, setting up the next portion of their counsel or message.