Where does the phrase "Be calm when you lose, be calm when you gain, strive for the inevitable, and let nature take its course" come from?

This sentence is a famous saying of Buddhism. So there is no specific source.

Be calm about loss: When things are gained, there will be losses. Don't worry about losing things.

Be calm about gain: When you get something, you must be fair and honest, be worthy of others, and do not harm others to benefit yourself.

The above two can also be called "don't worry about gains and losses." We must have a normal mind when dealing with gains and losses, and reduce our excessive desire for external things.

Striving for the inevitable: refers to me Only when there is a certain reason can we insist on it. In other words, the only thing that can make me argue and stick to the truth is the truth. Don't argue forcefully, and don't argue for three points unreasonably. You should be upright and upright, defend if you have reason, admit it if it is unreasonable, and listen to reason with an open mind.

Let nature take its course: It means that all things have their own reasons and things in the world cannot be forced. Most of the time, it is up to people to plan things, and to do things naturally. As long as you do your best, there is nothing to be ashamed of. For many experiences and results , we should face it with an optimistic and relaxed mood, and always have a normal heart.