Khandogya-Upanishad: Fourth Prapathaka, Ninth Khanda

1. Thus he reached the house of his teacher. The teacher said to him: “Satyakama.” He replied: “Sir.”

2. 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;

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