BṚHADĀRAṆYAKA UPANISHAD · 1.2.2

आपो वा अर्कः ।
तद् यद् अपां शर आसीत् तत् समहन्यत ।
सा पृथिव्य् अभवत् ।
तस्याम् अश्राम्यत् ।
तस्य श्रान्तस्य तप्तस्य तेजोरसो निरवर्तताग्निः ॥

āpo vā arkaḥ ।
tad yad apāṃ śara āsīt tat samahanyata ।
sā pṛthivy abhavat ।
tasyām aśrāmyat ।
tasya śrāntasya taptasya tejoraso niravartatāgniḥ ॥

Verily water is arka. And what was there as the froth of the water, that was hardened, and became the earth. On that earth he (Death) rested, and from him, thus resting and heated, Agni (Virâg) proceeded, full of light.

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