BṚHADĀRAṆYAKA UPANISHAD · 4.3.7

कतम आत्मेति -- यो ऽयं विज्ञानमयः प्राणेषु हृद्य् अन्तर्ज्योतिः पुरुषः ।
स समानः सन्न् उभौ लोकाव् अनुसंचरति ध्यायतीव लेलायतीव ।
स हि स्वप्नो भूत्वेमं लोकम् अतिक्रामति मृत्यो रूपाणि ॥

katama ātmeti -- yo ’yaṃ vijñānamayaḥ prāṇeṣu hṛdy antarjyotiḥ puruṣaḥ ।
sa samānaḥ sann ubhau lokāv anusaṃcarati dhyāyatīva lelāyatīva ।
sa hi svapno bhūtvemaṃ lokam atikrāmati mṛtyo rūpāṇi ॥

Ganaka Vaideha said: 'Who is that Self?' Yâgñavalkya replied: 'He who is within the heart, surrounded by the Prânas (senses), the person of light, consisting of knowledge. He, remaining the same, wanders along the two worlds, as if thinking, as if moving. During sleep (in dream) he transcends this world and all the forms of death (all that falls under the sway of death, all that is perishable).

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