Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 1.3.5

1.3.5

atha ha śrotramūcustvaṃ na udgāyeti tatheti tebhvaḥ śrotramudagāyadyaḥśrotre bhogastaṃ devebhya āgāyadyatkalyāṇam̐ śṛṇoti tadātmane | te viduranena vai na udgātrātyeṣyantīti tamabhidrutya pāpmanāvidhyansa yaḥ sa pāpmā yadevedamapratirūpam̐ śṛṇoti sa eva sa pāpmā

Then they said to the ear: 'Do thou sing out for us.' 'Yes,' said the ear, and sang. Whatever delight there is in the ear, that he obtained for the Devas by singing; but that he heard well, that was for himself. The Asuras knew: 'Verily, through this singer they will overcome us.' They therefore rushed at the singer, and pierced him with evil. That evil which consists in hearing what is bad, that is that evil.