Bhagwad Gita
Chapter 4, Verse 17
कर्मणो ह्यपि बोद्धव्यं बोद्धव्यं च विकर्मणः।
अकर्मणश्च बोद्धव्यं गहना कर्मणो गतिः।।4.17।।
The intricacies of action are very hard to understand. Therefore one should know properly what action is, what forbidden action is, and what inaction is.