Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 4.3.6

4.3.6

astamita āditye yājñavalkya candramasyastamite śānte'gnau śāntāyāṃ vāci kiṃjyotirevāyaṃ puruṣa ityātmaivāsya jyotirbhavatītyātmanaivāyaṃ jyotiṣāste palyayate karma kurute vipalyetīti

Ganaka Vaideha said: 'When the sun has set, O Yâgñavalkya, and the moon has set, and the fire is gone out, and the sound hushed, what is then the light of man?'
Yâgñavalkya said: 'The Self indeed is his light; for, having the Self alone as his light, man sits, moves about, does his work, and returns.'