Chapter 18 · verse 32 · gita.yoga.org

Bhagavad Gita 18.32

अधर्मं धर्ममिति या मन्यते तमसाऽऽवृता।सर्वार्थान्विपरीतांश्च बुद्धिः सा पार्थ तामसी

That intellect, O Arjuna, which is enveloped in darkness and sees Adharma as Dharma and all things perverted, is Tamasic (dark).