KAṬHA UPANISHAD · 1.2.7

श्रवणायापि बहुभिर्यो न लभ्यः
शृण्वन्तोऽपि बहवो यं न विद्युः ।
आश्चर्यो वक्ता कुशलोऽस्य लब्धा
आश्चर्यो ज्ञाता कुशलानुशिष्टः ॥ ७॥

śravaṇāyāpi bahubhiryo na labhyaḥ
śṛṇvanto’pi bahavo yaṃ na vidyuḥ ।
āścaryo vaktā kuśalo’sya labdhā
āścaryo jñātā kuśalānuśiṣṭaḥ ॥ 7॥

He that is not easy even to be heard of by many, and even of those that have heard they are many who have not known Him, — a miracle is the man that can speak of Him wisely or is skilful to win Him, and when one is found, a miracle is the listener who can know God even when taught of Him by the knower.

Sri Aurobindo · CWSA 18, p. 109