Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 6.2.7

6.2.7

sa hovāca vijñāyate hāsti hiraṇyasyāpāttaṃ go aśvānāṃ dāsīnāṃ pravārāṇāṃ paridhānasya mā no bhavānbahoranantasyāparyantasyābhyavadānyo bhūditi sa vai gautama tīrthenecchāsā ityupaimyahaṃ bhavantamiti vācā ha smaiva pūrva upayanti sa hopāyanakīrtyovāsa

He said: 'You know well that I have plenty of gold, plenty of cows, horses, slaves, attendants, and apparel; do not heap on me what I have already in plenty, in abundance, and superabundance.'
The king said: 'Gautama, do you wish (for instruction from me) in the proper way?'
Gautama replied: 'I come to you as a pupil.'
In word only have former sages (though Brahmans) come as pupils (to people of lower rank), but Gautama actually dwelt as a pupil (of Pravâhana, who was a Râganya) in order to obtain the fame of having respectfully served his master.