CHĀNDOGYA UPANISHAD · 1.2.5

अथ ह श्रोत्रम् उद्गीथम् उपासां चक्रिरे ।
तद् धासुराः पाप्मना विविधुः ।
तस्मात् तेनोभयं शृणोति श्रवणीयं चाश्रवणीयं च ।
पाप्मना ह्य् एतद् विद्धम् ॥

atha ha śrotram udgītham upāsāṃ cakrire ।
tad dhāsurāḥ pāpmanā vividhuḥ ।
tasmāt tenobhayaṃ śṛṇoti śravaṇīyaṃ cāśravaṇīyaṃ ca ।
pāpmanā hy etad viddham ॥

Then they meditated on the udgîtha (Om) as the ear, but the Asuras pierced it with evil. Therefore we hear both what should be heard and what should not be heard. For the car is pierced by evil.

F. Max Müller · The Upanishads, Sacred Books of the East 1 (1879)