Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 1.2.6

1.2.6

so'kāmayata bhūyasā yajñena bhūyo yajeyeti | so'śrāmyatsa tapo'tapyata tasya śrāntasya taptasya yaśo vīryamudakrāmat | prāṇā vai yaśo vīryaṃ tatprāṇeṣūtkrānteṣu śarīram̐ śvayitumadhriyata tasya śarīra eva mana āsīt

He desired to sacrifice again with a greater sacrifice. He toiled and performed penance. And while he toiled and performed penance, glorious power went out of him. Verily glorious power means the senses (prâna). Then when the senses had gone out, the body took to swelling (sva-yitum), and mind was in the body.