BṚHADĀRAṆYAKA UPANISHAD · 4.3.4

अस्तमित आदित्ये याज्ञवल्क्य चन्द्रमस्य् अस्तमिते किंज्योतिर् एवायं पुरुष इति ।
अग्निर् एवास्य ज्योतिर् भवतीति ।
अग्निनैव ज्योतिषास्ते पल्ययते कर्म कुरुते विपल्येतीति ।
एवम् एवैतद् याज्ञवल्क्य ॥

astamita āditye yājñavalkya candramasy astamite kiṃjyotir evāyaṃ puruṣa iti ।
agnir evāsya jyotir bhavatīti ।
agninaiva jyotiṣāste palyayate karma kurute vipalyetīti ।
evam evaitad yājñavalkya ॥

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

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