The Long Āgama Sūtra 25.18

25.18

佛言：「迦葉！此是世間不共法。所謂沙門法、婆羅門法難，迦葉！乃至優婆夷亦能知此法。離服倮形，乃至無數方便苦役此身，但不知其心：為有恚心、為無恚心，有恨心、無恨心，有害心、無害心？若知此心者，不名沙門、婆羅門，為已不知故，沙門、婆羅門為難。」

The Buddha said, “Kāśyapa, this is world’s special teaching, that the teachings of ascetics and priests are difficult. Kāśyapa, even a laywoman can understand these teachings. Parting with clothing and going naked … tormenting one’s body with these countless hardships, a person still doesn’t understand their mind: ‘Am I angry or not … resentful or not … harmful or not?’ If someone knows these mental states, they aren’t named ascetics or priests. It’s because they don’t understand themselves that being an ascetic or priest is difficult.”