Bhagavad Gita 4.18
He who can see inaction in action, and action in inaction, is the wisest among men. He is a saint, even though he still acts.
कर्मण्यकर्म यः पश्येदकर्मणि च कर्म यः।
स बुद्धिमान् मनुष्येषु स युक्तः कृत्स्नकर्मकृत्॥karmaṇyakarma yaḥ paśyedakarmaṇi ca karma yaḥ|
sa buddhimān manuṣyeṣu sa yuktaḥ kṛtsnakarmakṛt||
English translation (above): Shri Purohit Swami (1935), public domain. Sanskrit text via github.com/gita/gita (Unlicense).