Laozi was a famous thinker in the Spring and Autumn Period. Seeing the decline of the Zhou Dynasty, he left his native land and prepared to travel around Hanguguan. Yin, the great official guarding Hangu Pass, worships Lao Zi very much. I am very happy to hear that Lao Zi has come to Hangu Pass. However, when he knew that Laozi was going to travel abroad and felt sorry, he tried to keep Laozi. Therefore, Yin said to Lao Zi, "If you want to go through the customs, go through the customs, but keep your money."
Hearing this, Lao Tzu stayed in Hangu for a few days. A few days later, he gave Yin a book of about 5,000 words, and then rode away on a big blue cow. It is said that this work was later passed down as Tao Te Ching.