Brihadaranyaka Upanishad 4.3.5

4.3.5

astimita āditye yājñavalkya candramasyastamite śānte'gnau kiṃjyotirevāyaṃ puruṣa iti vāgevāsya jyotirbhavatīti vācaivāyaṃ jyotiṣāste palyayate karma kurute vipalyetīti tasmādvai samrāḍapi yatra svaḥ pāṇirna vinirjñāyati atha yatra vāguccaratyupaiva tatra nyetītyevamevaitadyājñavalkya

Ganaka Vaideha said: 'When the sun has set, O Yâgñavalkya, and the moon has set, and the fire is gone out, what is then the light of man?'
Yâgñavalkya replied: 'Sound indeed is his light; for, having sound alone for his light, man sits, moves about, does his work, and returns. Therefore, O King, when one cannot see even one's own hand, yet when a sound is raised, one goes towards it.'
Ganaka Vaideha said: 'So indeed it is, O Yâgñavalkya.'