Patanjali Yoga Sutra 2.5

Sanskrit Sutra

अनित्याशुचि-दुःखानात्मसु नित्य-शुचि-सुखात्मख्यातिरविद्या॥५॥

Swami Vivekananda's translation

Ignorance is taking the non-eternal, the impure, the painful, and the non-Self for the eternal, the pure, the happy, and the Atman or Self (respectively).

Chapter link: Yoga Sutra Chapter 2