Shatapatha Brahmana 11.4.1.9

11.4.1.9

sa hovāca yathā brahmā brahmaputro mūrdhāsya vipatedya enamupavalheteti te ha tata eva vipreyustaṃ ha tata eva samitprāṇiḥ praticakram upa tvāyānīti kimadhyeṣyamāṇa iti yāneva mā praśnānaprākṣīstāneva me vibrūhīti sa hovācānupetāyaiva ta enānbravāṇīti

He said, 'Even as a Brahman, and the son of a Brahman: the head would fly off of whosoever should (dare to) challenge him to a disputation.' They then went away in all directions. He (Uddâlaka) then came back to him, with fire-wood in his hand, and said, 'I want to become thy pupil.'--'What wouldst thou study?'--'Even those questions which thou didst ask me--explain them to me!' He said, 'I will tell them to thee even without thy becoming my pupil.'