CHĀNDOGYA UPANISHAD · 3.11.5
इदं वाव तज् ज्येष्ठाय पुत्राय पिता ब्रह्म प्रब्रूयात् प्रणाय्याय वान्तेवासिने ॥
idaṃ vāva taj jyeṣṭhāya putrāya pitā brahma prabrūyāt praṇāyyāya vāntevāsine ॥
A father may therefore tell that doctrine of Brahman to his eldest son, or to a worthy pupil.
F. Max Müller · The Upanishads, Sacred Books of the East 1 (1879)