佛祖統紀 35.84

35.84

In the first year, Laozi entered Hangu Pass heading west. Yin Xi, the magistrate of the pass, saw purple clouds moving westward and knew that an enlightened person was due to pass through. He welcomed him and presented offerings of courtesy. For Yin Xi, Laozi composed the Daode jing in five thousand words. Going west to the flowing sands, it is not known where he ended. Yin Xi composed a book in nine chapters entitled Guanlingzi.