一日千里 (By leaps and bounds)
Meaning: that horse run very fast, runs one of a thousand miles a day, now describe the progress of people or things quickly develop an extremely rapid rate.
Story: During Warring States Period, Prince Dan (太子丹) of Yan (燕国) was kept as hostage in Zhao along with Ying Zheng (嬴政) from Qin (秦国). They kept along well with each other.
Later, Ying Zheng returned to Qin and kept Prince Dan as Qin’s hostages. He did not give any special care to Prince Dan, but treated Dan bad and made things difficult for him. Prince found an opportunity and ran back to the Yan. After returning to his home, Prince Dan would like to revenge on Ying Zheng. However, Yan was such a small country and weak in strength, Prince Dan realized that it was impossible to carry out the revenge.
Soon, Qin sent troops to attack Qi (齐国), Chu (楚国), Han (韩国), Wei (魏国), Zhao (赵国) and other countries, and gradually approached Yan. Yan was very afraid of the monarch. Prince Dan was also very sad and turned to his teacher Ju Wu (鞠武) for help. Ju Wu said: “I have a good friend of mine, named Tian Guang (田光), he was very witty, Strategies, and you can discuss with him.”
Prince Dan treated Tian Guang with respect and said, “We hope you can help to resist Qin’s invasion.”
Tian Guang listened silently, holding the hand of Prince Dan and go outside. He pointed a horse in the yard and told a story to Dan. “There was a fine horse which could run more than a thousand miles a day. But when it is old, a bad horse can be run faster than it. Do you know the reason?” Dan Prince said: “That is because the horse does not have the energy to do so.”
“Yes! – All the stories you’ve heard about me happened in my youth and prime of life. But I am old now, I do not have the energy” Tian Guang went on saying “Although I am not able to help you, I would like to recommend my good friend Jing Ke (荆轲) who can shoulder this responsibility.
Later, Prince Dan made Jing Ke assassinate Ying Zheng of Qin, but ended with failure.
Source: 《史记·刺客列传》臣闻骐骥盛壮之时，—日而驰千里；至其衰老；驾马先之。