Chapter 18 · Liberation through Renunciation · Krishna said
Bhagavad Gita 18.35 — Tamasic resolve
यया स्वप्नं भयं शोकं विषादं मदमेव च। न विमुञ्चति दुर्मेधा धृतिः सा पार्थ तामसी ॥
yayā svapnaṃ bhayaṃ śokaṃ viṣādaṃ madameva ca | na vimuñcati durmedhā dhṛtiḥ sā pārtha tāmasī ||
Meaning
The resolve by which a dull person refuses to let go of sleep, fear, grief, gloom and arrogance is tamasic.