Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 5.5.2

5.5.2

tadyattatsatyamasau sa ādityo ya eṣa etasminmaṇḍale puruṣo yaścāyaṃ dakṣiṇe'kṣanpuruṣastāvetāvanyonyasminpratiṣṭhitau raśmibhireṣo'sminpratiṣṭhitaḥ prāṇairayamamuṣmin sa yadotkramiṣyanbhavati śuddhamevaitanmaṇḍalaṃ paśyati nainamete raśmayaḥ pratyāyanti

Now what is the true, that is the Âditya (the sun), the person that dwells in yonder orb, and the person in the right eye. These two rest on each other, the former resting with his rays in the latter, the latter with his prânas (senses) in the former. When the latter is on the point of departing this life, he sees that orb as white only, and those rays (of the sun) do not return to him.