Bhagavad Gita · 15.20

इति गुह्यतमं शास्त्रमिदमुक्तं मयानघ।एतद्बुद्ध्वा बुद्धिमान्स्यात्कृतकृत्यश्च भारत॥

iti guhyatamaṁ śhāstram idam uktaṁ mayānagha etad buddhvā buddhimān syāt kṛta-kṛtyaśh cha bhārata

Thus, this most secret teaching has been revealed by me, O sinless one. Understanding this, a person becomes truly wise and fulfills all their duties, O Arjuna.

— Bhagavad Gita 15.20

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