Chāndogya Upaniṣad 6.8.2

6.8.2

sa yathā śakuniḥ sūtreṇa prabaddho diśaṃ diśaṃ patitvānyatrāyatanam alabdhvā bandhanam evopaśrayate \
evam eva khalu somya tan mano diśaṃ diśaṃ patitvānyatrāyatanam alabdhvā prāṇam evopaśrayate \
prāṇabandhanaṃ hi somya mana iti \\608.2\

1. Then Uddālaka Āruṇi said to Śvetaketu, his son: ‘Understand from me, my dear, the condition of sleep. When a person here sleeps (*svapiti*), as it is called, then, my dear, he has reached Being, he has gone to his own (*svam apīta*). Therefore they say of him “he sleeps”; for he has gone to his own. 2. As a bird fastened with a string, after flying in this direction and in that without finding an abode elsewhere, rests down just upon its fastening—even so, my dear, the mind, after flying in this direction and in that without finding an abode elsewhere, rests down just upon breath; for the mind, my dear, has breath as its fastening.