Now there was on that spot in old times a great and noble city called Cambaluc [Beijing], which is as much as to say in our tongue "The city of the Emperor." But the Great Kaan was informed by his Astrologers that this city would prove rebellious, and raise great disorders against his imperial authority. So he caused the present city to be built close beside the old one, with only a river between them. And he caused the people of the old city to be removed to the new town that he had founded; and this is called Taidu.
Source:
Polo, Marco: The Travels of Marco Polo. Book 2, Chapter 11: Concerning the city of Cambaluc.
Further Reading:
When exactly did this all happen?
The reason I ask is because I am wondering if it is related to a comet sighting. Various civilisations throughout history have considered comet sightings as a sign that something bad is going to happen. I am thinking the reasoning could be the appearance of the Great Comet of 1264, which was visible in China.