Bhagavad Gita - Chapter 2, Verse 47

Karmanye Vadhikaraste Ma Phaleshu Kadachana
Ma Karma Phala Hetur Bhur Ma Te Sangostvakarmani

Translation: You have the right to perform your prescribed duties, but you are not entitled to the fruits of your actions. Never consider yourself the cause of the results of your activities, and never be attached to not doing your duty.

This single verse from the Bhagavad Gita encapsulates the essence of Karma Yoga. Lord Krishna instructs Arjuna on the battlefield of Kurukshetra to perform his duty without attachment to the results. This teaching forms the philosophical foundation of selfless service (seva) in Hindu thought and has influenced thinkers worldwide, including Mahatma Gandhi who considered it his guiding principle.