Chandogya Upanishad 3.41

इदं वाव तज्ज्येष्ठाय पुत्राय पिता ब्रह्म प्रब्रूयात्प्रणाय्याय वान्तेवासिने ॥ ५ ॥

3.11.5 — idam vava tajjyeshthaya putraya pita brahma prabruyatpranayyaya vantevasine || 5 ||

A father may therefore tell that doctrine of Brahman to his eldest son, or to a worthy pupil. But no one should tell it to anybody else, even if he gave him the whole sea-girt earth, full of treasure, for this doctrine is worth more than that, yea, it is worth more.

पिता यह ब्रह्म-विद्या अपने ज्येष्ठ पुत्र अथवा योग्य अन्तेवासी शिष्य को ही पढ़ावे —