Chandogya Upanishad 4.9

prāpa hācaryakulam tam ācaryo 'bhyuvāda satyakāma 3 iti bhagava iti ha pratiśuśrāva

Thus he reached the house of his teacher. The teacher said to him: 'Satyakâma.' He replied: 'Sir.'

brahmavid iva vai somya bhāsi ko nu tvānuśaśāseti anye manuṣyebhya iti ha pratijajñe bhagavāṃs tv eva me kāme brūyāt

The teacher said: 'Friend, you shine like one who knows Brahman. Who then has taught you?' He replied: 'Not men. But you only, Sir, I wish, should teach me;

śrutaṃ hy eva me bhagavaddṛśebhya ācāryāddhaiva vidyā viditā sādhiṣṭhaṃ prāpatīti tasmai ha etad eva uvāca atra ha na kiṃcana vīyāyeti vīyāyeti

'For I have heard from men like you, Sir, that only knowledge which is learnt from a teacher (Âkârya), leads to real good.' Then he taught him the same knowledge. Nothing was left out, yea, nothing was left out.