Bhagavad Gita 4.34
This you shall learn by prostrating yourself at the master's feet, questioning them, and serving them. The wise who have realized the truth will teach you wisdom.
तद्विद्धि प्रणिपातेन परिप्रश्नेन सेवया।
उपदेक्ष्यन्ति ते ज्ञानं ज्ञानिनस्तत्त्वदर्शिनः॥tadviddhi praṇipātena paripraśnena sevayā|
upadekṣyanti te jñānaṃ jñāninastattvadarśinaḥ||
English translation (above): Shri Purohit Swami (1935), public domain. Sanskrit text via github.com/gita/gita (Unlicense).