Brihadaranyaka Upanishad 4.3.3

4.3.3

astamita ādityeyājñavalkya kiṃjotirevāyaṃ puruṣa iti candramā evāsya jyotirbhavatīti candramasaivāyaṃ jyotiṣāste palyayate karma kurute vipalyetītyevamevaitadyājñavalkya

Ganaka Vaideha said: 'When the sun has set, O Yâgñavalkya, what is then the light of man?'
Yâgñavalkya replied: 'The moon indeed is his light; for, having the moon alone for his light, man sits, moves about, does his work, and returns.'
Ganaka Vaideha said: 'So indeed it is, O Yâgñavalkya.'