Bhagavad Gita 18.22
But that which clings blindly to one idea as if it were all, without logic, truth, or insight, has its origin in darkness.
यत्तु कृत्स्नवदेकस्मिन्कार्ये सक्तमहैतुकम्।अतत्त्वार्थवदल्पं च तत्तामसमुदाहृतम्॥yattu kṛtsnavadekasminkārye saktamahaitukam|atattvārthavadalpaṃ ca tattāmasamudāhṛtam||
English translation (above): Shri Purohit Swami (1935), public domain. Sanskrit text via github.com/gita/gita (Unlicense).