Bhagavad Gita · 18.32

अधर्मं धर्ममिति या मन्यते तमसावृता।सर्वार्थान्विपरीतांश्च बुद्धिः सा पार्थ तामसी॥

adharmaṁ dharmam iti yā manyate tamasāvṛitā sarvārthān viparītāṁśh cha buddhiḥ sā pārtha tāmasī

O Partha, that intellect which, enveloped in darkness, regards adharma as dharma and sees all things in a perverted way — that intellect is tamasic.

— Bhagavad Gita 18.32

Source: Sacred-Texts.com (Sanskrit) + Bhagavad-Gita.io (translations)