ŚVETĀŚVATARA UPANISHAD · 1.13

वह्नेर् यथा योनिगतस्य मूर्तिर् न दृश्यते नैव च लिङ्गनाशः ।
स भूय एवेन्धनयोनिगृह्यस् तद्वोभयं वै प्रणवेन देहे ॥

vahner yathā yonigatasya mūrtir na dṛśyate naiva ca liṅganāśaḥ ।
sa bhūya evendhanayonigṛhyas tadvobhayaṃ vai praṇavena dehe ॥

As the form of fire, while it exists in the under-wood, is not seen, nor is its seed destroyed, but it has to be seized again and again by means of the stick and the under-wood, so it is in both cases, and the Self has to be seized in the body by means of the pranava (the syllable Om).

F. Max Müller · The Upanishads, Sacred Books of the East 15 (1884)