BṚHADĀRAṆYAKA UPANISHAD · 4.3.5

अस्तमित आदित्ये याज्ञवल्क्य चन्द्रमस्य् अस्तमिते शान्ते ऽग्नौ किंज्योतिर् एवायं पुरुष इति ।
वाग् एवास्य ज्योतिर् भवतीति ।
वाचैवायं ज्योतिषास्ते पल्ययते कर्म कुरुते विपल्येति ।
तस्माद् वै सम्राड् अपि यत्र स्वः पाणिर् न विनिर्ज्ञायते ऽथ यत्र वाग् उच्चरत्य् उपैव तत्र न्येतीति ।
एवम् एवैतद् याज्ञवल्क्य ॥

astamita āditye yājñavalkya candramasy astamite śānte ’gnau kiṃjyotir evāyaṃ puruṣa iti ।
vāg evāsya jyotir bhavatīti ।
vācaivāyaṃ jyotiṣāste palyayate karma kurute vipalyeti ।
tasmād vai samrāḍ api yatra svaḥ pāṇir na vinirjñāyate ’tha yatra vāg uccaraty upaiva tatra nyetīti ।
evam evaitad yājñavalkya ॥

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.'

F. Max Müller · The Upanishads, Sacred Books of the East 15 (1884)