CHĀNDOGYA UPANISHAD · 4.9.3

श्रुतं ह्य् एव मे भगवद्दृशेभ्य आचार्याद् धैव विद्या विदिता साधिष्ठं प्रापतीति ।
तस्मै ह एतद् एव उवाच ।
अत्र ह न किंचन वीयायेति वीयायेति ॥

śrutaṃ hy eva me bhagavaddṛśebhya ācāryād dhaiva 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.

F. Max Müller · The Upanishads, Sacred Books of the East 1 (1879)