Katha Upanishad Verse 2.2.9

Yama uses the analogy of fire to illustrate how the one Atman assumes diverse forms according to the bodies it enters. Just as fire, though one, takes the shape of the objects it burns, the internal Atman of all living things assumes a form for every form while remaining independent.

Sanskrit Verse

अग्निर्यथैको भुवनं प्रविष्टो रूपं रूपं प्रतिरूपो बभूव ।
एकस्तथा सर्वभूतान्तरात्मा रूपं रूपं प्रतिरूपो बहिश्च ॥ ९ ॥

Transliteration

agniryathaiko bhuvanaṃ praviṣṭo rūpaṃ rūpaṃ pratirūpo babhūva |
ekastathā sarvebhūtāntarātmā rūpaṃ rūpaṃ pratirūpo bahiśca || 9 ||

Translation

As fire, though one, having entered the world, takes a separate form in respect of every form, so does the internal atman of all living things assume a form for every form and is outside all forms.