Chapter 9 · verse 2 · gita.yoga.org

Bhagavad Gita 9.2

राजविद्या राजगुह्यं पवित्रमिदमुत्तमम्। प्रत्यक्षावगमं धर्म्यं सुसुखं कर्तुमव्ययम्

This is the royal science, the royal secret, the supreme purifier, realizable by direct intuitive knowledge, according to righteousness, very easy to perform and imperishable.