KAṬHA UPANISHAD · 2.2.9

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

agniryathaiko bhuvanaṃ praviṣṭo
rūpaṃ rūpaṃ pratirūpo babhūva ।
ekastathā sarvabhūtāntarātmā
rūpaṃ rūpaṃ pratirūpo bahiśca ॥ 9॥

Even as one Fire hath entered into the world but it shapeth itself to the forms it meeteth, so there is one Spirit within all creatures but it shapeth itself to form and form; it is likewise outside these.