Bhagavad Gita · 9.2

राजविद्या राजगुह्यं पवित्रमिदमुत्तमम्।प्रत्यक्षावगमं धर्म्यं सुसुखं कर्तुमव्ययम्॥

rāja-vidyā rāja-guhyaṁ pavitram idam uttamam pratyakṣhāvagamaṁ dharmyaṁ su-sukhaṁ kartum avyayam

This knowledge is the king of all knowledge, the king of all secrets. It is supremely purifying, directly perceivable, righteous, joyful to practice, and imperishable.

— Bhagavad Gita 9.2

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