Muṇḍaka Upaniṣad 3.2.4

3.2.4

nāyam ātmā balahīnena labhyo na ca pramādāt tapaso vāpy aliṅgāt.
etair upāyair yatate yas tu vidvāṃs tasyaiṣa ātmā viśate brahma dhāma.

4. This Soul (Ātman) is not to be obtained by one destitute of
fortitude,
Nor through heedlessness, nor through a false notion of
austerity (*tapas*).
But he who strives by these means, provided he knows—
Into his Brahma-abode this Soul (Ātman) enters.