CHĀNDOGYA UPANISHAD · 1.3.1

अथाधिदैवतम् ।
य एवासौ तपति तम् उद्गीथम् उपासीत ।
उद्यन् वा एष प्रजाभ्य उद्गायति ।
उद्यंस् तमो भयम् अपहन्ति ।
अपहन्ता ह वै भयस्य तमसो भवति य एवं वेद ॥

athādhidaivatam ।
ya evāsau tapati tam udgītham upāsīta ।
udyan vā eṣa prajābhya udgāyati ।
udyaṃs tamo bhayam apahanti ।
apahantā ha vai bhayasya tamaso bhavati ya evaṃ veda ॥

Now follows the meditation on the udgîtha with reference to the gods. Let a man meditate on the udgîtha (Om) as he who sends warmth (the sun in the sky). When the sun rises it sings as Udgâtri for the sake of all creatures. When it rises it destroys the fear of darkness. He who knows this, is able to destroy the fear of darkness (ignorance).

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