These iconic lines are a sloka from the Bhagavad Gita, Chapter 2, verse 47.
In the great epic Mahabharat, the Pandavas and the Kauravas struggle for dominance that led to the great Kurukshetra war. At the start of the war, the great Pandava warrior Arjun was reluctant to fight. He lamented to Lord Krishna that he would become a sinner if he killed his own gurus, grandfather and other kin, all for the mere gain of a kingdom. Lord Krishna reproached him for displaying such immaturity and explained the Sankya Yog, the philosophies of the mortal body, and the eternal Atma. That’s when Lord Krishna spoke these lines.
कर्मण्येवाधिकारस्ते मा फलेषु कदाचन ।
मा कर्मफलहेतुर्भुर्मा ते संगोऽस्त्वकर्मणि ॥
karmanye vadhikaraste, ma phaleshou kada chana
ma karma phala heturbhuhu matey sangostva akarmani
