CHĀNDOGYA UPANISHAD · 1.2.6

अथ ह मन उद्गीथम् उपासां चक्रिरे ।
तद् धासुराः पाप्मना विविधुः ।
तस्मात् तेनोभयं संकल्पयते संकल्पनीयं च ।
पाप्मना ह्य् एतद् विद्धम् ॥

atha ha mana udgītham upāsāṃ cakrire ।
tad dhāsurāḥ pāpmanā vividhuḥ ।
tasmāt tenobhayaṃ saṃkalpayate saṃkalpanīyaṃ ca ।
pāpmanā hy etad viddham ॥

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

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