Bhagavad-Gita is one of the holy books of Hindus and worlds largest reading book. On it, Lord Krishna makes a speech to Arjuna where Arjuna is in a religious war and fought against the iniquity of Kauravs who was their cousin-brothers. And when Arjun felt guilty to fight against their relatives.
Then Lord Krishna convinced Arjuna to fight for the truth. And give him(the human beings) advice.
Then Lord Krishna replied
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Get lesson from Bhagavad Gita...... Part one:.... Arjuna–Visada yoga
Perform your duty equi-poised, O Arjuna, abandoning all attachment to success or failure. Such equanimity is called yoga.
Lord krishna on Arjuna at the war field
11 BRIEF LESSONS FROM THE BHAGAVAD GITA FOR BRINGING PEACE INTO OUR LIVES