Bhagavad Gita · 4.17

कर्मणो ह्यपि बोद्धव्यं बोद्धव्यं च विकर्मणः।अकर्मणश्च बोद्धव्यं गहना कर्मणो गतिः॥

karmaṇo hy api boddhavyaṁ boddhavyaṁ cha vikarmaṇaḥ akarmaṇaśh cha boddhavyaṁ gahanā karmaṇo gatiḥ

You must understand the nature of action, the nature of forbidden action, and the nature of inaction. The path of action is profound and difficult to understand.

— Bhagavad Gita 4.17

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