Brihadaranyaka Upanishad 4.3.1

4.3.1

janakam̐ ha vaidehaṃ yājñavalkyo jagāma sa mene na vadiṣya ityatha ha yajjanakaśca vaideho yājñavalkyaścāgnihotre samūdāte tasmai ha yājñavalkyo varaṃ dadau sa ha kāmapraśnameva vabre tam̐ hāsmai dadau tam̐ ha samrāḍeva pūrvaṃ papraccha

Yâgñavalkya came to Ganaka Vaideha, and he did not mean to speak with him. But when formerly Ganaka Vaideha and Yâgñavalkya had a disputation on the Agnihotra, Yâgñavalkya had granted him a boon, and he chose (for a boon) that he might be free to ask him any question he liked. Yâgñavalkya granted it, and thus the King was the first to ask him a question.