Muṇḍaka Upaniṣad 2.2.3

2.2.3

dhanur gṛhītvaupaniṣadaṃ mahāstraṃ śaraṃ hy upāsāniśitaṃ sandhayīta.
āyamya tad bhāvagatena cetasā lakṣyaṃ tad evākṣaraṃ somya viddhi.

3. Taking as a bow the great weapon of the Upanishad,
One should put upon it an arrow sharpened by meditation.
Stretching it with a thought directed to the essence of That,
Penetrate that Imperishable as the mark, my friend.