CHĀNDOGYA UPANISHAD · 6.6.4

तेजसः सोम्याश्यमानस्य यो ऽणिमा स ऊर्ध्वः समुदीषति ।
सा वाग् भवति ॥

tejasaḥ somyāśyamānasya yo ’ṇimā sa ūrdhvaḥ samudīṣati ।
sā vāg bhavati ॥

'That which is the subtile portion of fire, when consumed, rises upwards, and becomes speech.

F. Max Müller · The Upanishads, Sacred Books of the East 1 (1879)