Chapter 18 · verse 35 · gita.yoga.org

Bhagavad Gita 18.35

यया स्वप्नं भयं शोकं विषादं मदमेव च।न विमुञ्चति दुर्मेधा धृतिः सा पार्थ तामसी

That firmness, O Arjuna, by which a stupid man does not abandon sleep, fear, grief, despair, and conceit, is Tamasic.