Bhagavad Gita 9.2
This is the supreme science, the supreme secret, the purest and best; it is intuitive, righteous, and to those who practice it, it is exceedingly pleasant.
राजविद्या राजगुह्यं पवित्रमिदमुत्तमम्।
प्रत्यक्षावगमं धर्म्यं सुसुखं कर्तुमव्ययम्॥rājavidyā rājaguhyaṃ pavitramidamuttamam|
pratyakṣāvagamaṃ dharmyaṃ susukhaṃ kartumavyayam||
English translation (above): Shri Purohit Swami (1935), public domain. Sanskrit text via github.com/gita/gita (Unlicense).