Srimad-Bhagavatam: Canto 11 - Chapter 1 - Verse 17


एत आत्महनोऽशान्ता अज्ञाने ज्ञानमानिन: ।सीदन्त्यकृतकृत्या वै कालध्वस्तमनोरथा: ॥ १७ ॥


eta ātma-hano ’śāntāajñāne jñāna-māninaḥsīdanty akṛta-kṛtyā vaikāla-dhvasta-manorathāḥ


The killers of the soul are never peaceful, because they consider that human intelligence is ultimately meant for expanding material life. Thus neglecting their real, spiritual duties, they are always in distress. They are filled with great hopes and dreams, but unfortunately these are always destroyed by the inevitable march of time.


There is a similar verse in Śrī Īśopaniṣad (3): “The killer of the soul, whoever he may be, must enter into the planets known as the worlds of the faithless, full of darkness and ignorance.”