Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 6.2.4

6.2.4

sa hovāca tathā nastvaṃ tāta jānīthā yathā yadahaṃ kiṃca veda sarvamahaṃ tattubhyamavocaṃ prehi tu tatra pratītya brahmacaryaṃ vatsyāva iti bhavāneva gacchatviti sa ājagāma gautamo yatrapravāhaṇasya jaivalerāsa tasmā āsanamā-hṛtyodakamāhārayāṃ kārātha hāsmā arghyaṃ cakāra tam̐ hovāca varaṃ bhagavate gautamāya dadma iti

The father said: 'You know me, child, that whatever I know, I told you. But come, we shall go thither, and dwell there as students.'
'You may go, Sir,' the son replied.
Then Gautama went where (the place of) Pravâhana Gaivali was, and the king offered him a seat, ordered water for him, and gave him the proper offerings. Then he said to him: 'Sir, we offer a boon to Gautama.'