KAṬHA UPANISHAD · 1.2.10

जानाम्यहं शेवधिरित्यनित्यं
न ह्यध्रुवैः प्राप्यते हि ध्रुवं तत् ।
ततो मया नाचिकेतश्चितोऽग्निः
अनित्यैर्द्रव्यैः प्राप्तवानस्मि नित्यम् ॥ १०॥

jānāmyahaṃ śevadhirityanityaṃ
na hyadhruvaiḥ prāpyate hi dhruvaṃ tat ।
tato mayā nāciketaścito’gniḥ
anityairdravyaiḥ prāptavānasmi nityam ॥ 10॥

I know of treasure that it is not for ever; for not by things unstable shall one attain That which is stable; therefore I heaped the fire of Nachiketas, and by the sacrifice of transitory things I won the Eternal.

Sri Aurobindo · CWSA 18, p. 110