Chapter 18 · Liberation through Renunciation · Krishna said
Bhagavad Gita 18.22 — Tamasic knowledge
यत्तु कृत्स्नवदेकस्मिन्कार्ये सक्तमहैतुकम्। अतत्त्वार्थवदल्पं च तत्तामसमुदाहृतम् ॥
yattu kṛtsnavadekasminkārye saktamahaitukam | atattvārthavadalpaṃ ca tattāmasamudāhṛtam ||
Meaning
The knowledge that clings to one small thing as if it were everything, without reason, without truth and narrow, is called tamasic.