Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 1.3.2

1.3.2

te ha vācamūchustvaṃ na udgāyeti tatheti tebhyo vāgudagāyat | yo vāci bhogastaṃ devebhya āgāyadyatkalyāṇaṃ vadati tadātmane | te viduranena vai na udgātrātyeṣyantīti tamabhidrutya pāpmanāvidhyansa yaḥ sa pāpmā yadevedamapratirūpaṃ vadati sa eva sa pāpmā

They said to speech (Vâk): 'Do thou sing out for us (the udgîtha).' 'Yes,' said speech, and sang (the udgîtha). Whatever delight there is in speech, that she obtained for the Devas by singing (the three pavamânas); but that she pronounced well (in the other nine pavamânas), that was for herself. The Asuras knew: 'Verily, through this singer they will overcome us.' They therefore rushed at the singer and pierced her with evil. That evil which consists in saying what is bad, that is that evil.