Kaṭha Upaniṣad 2.10

2.10

jānāmy ahaṃ śevadhir ity anityaṃ na hy adhruvaiḥ prāpyate hi dhruvaṃ tat /
tato mayā nāciketaś cito 'gnir anityair dravyaiḥ prāptavān asmi nityam //

10. I know that what is known as treasure is something inconstant.
For truly, that which is steadfast is not obtained by those
who are unsteadfast.
Therefore the Naciketas-fire has been built up by me,
And with means which are inconstant I have obtained that
which is constant.